Cooking Up Contingency Plans

So this sent me down an absolute rabbit hole. As a DM there's a few ways I'd consider to stop this being entirely game-breaking:

You could argue that the only thing strength before death shows is that you can activate strength before death between hitting 0 and getting knocked out. A wizard is no samurai. Therefore concentration spells are not allowed.
You could argue that life steal requires life to steal, and as such you can't life steal yourself.
You could enforce the requirement of the figurine required for vampiric touch, then engineer a scenario to remove it at a critical moment and see if they realise.

Personally I would instead depart from RAW and point out a version of option 2, but a lenient one. Something like "you can do this but you are sapping your very essence to do it. Every time you do it, you permanently lose 10% of your HP" or "every time you do this you increase the number of death saving throws you must succeed before you die". Or my personal favourite: "every time you do this you perturb the very laws of nature. Nature is rather fond of its laws and so decides to perturb you right back. Roll on this table to see what happens." and make the table include the above alongside a few other things and maybe a roll on the wild magic table.

In the end I enjoy ingenuity but the role of DM gives you a lot of latitude to... handle... those who believe they found a loophole.

Nature is rather fond of its laws and so decides to perturb you right back.

"Your character has started to understand and unlock the secrets of undeath. Do this again and you may need to find a phylactery quick, because each time moves you one step closer to becoming a lich"
It goes off but doesn't heal you because the text specifically says

The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.
- That's flavour text.
  
  The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds. Make a melee spell attack against a creature within your reach. On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt. Until the spell ends, you can make the attack again on each of your turns as an action.
  
  You are, in fact, a creature within your reach.
- The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.
  
  Doesn't say "exclusively from others". Without casting this spell you couldn't do that normally.
  
  It also says,
  
  On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.
  
  No qualifiers about the target having to be a creature other than you. It just has to be a creature within your reach.
  
  Also as regards:
  
  As a DM there’s a few ways I’d consider to stop this being entirely game-breaking:
  
  Do you also ban death ward and healing word? What about wizards who dip a level into virtually any other spellcaster or take a feat for a healing spell who can do this kind of thing without even having to make an attack roll or take necrotic damage?
I mean, there's always a way to houserule something to fix the broken rules. Unless you're DMing an AL game.
- Yup, in my experience the best way to beat a rules lawyer is to be a better and funnier rulemaker.

Unfortunately:

half of zero is still zero, and
since one can't go below 0HP there's no HP pool to take health from.

As someone who had to DM more than I'd like, no dice, mate.

You're still dealing damage, e.g. it could kill you from the massive damage rule if you dealt enough. Vampiric touch counts how much damage you deal, not how much hp the target loses.
- Not to mention I'd allow on creativity and rule of cool. I want players like that in my table.

since there is no negative HP you are doing no damage past 0. So even if the spell hits it does no damage since there is no damage left to do, half of 0 is 0.

As I've pointed out in other comments there's a specific rule for damage past 0 HP. It's called “Damage at 0 Hit Points.” under the Death Saving Throws section, and it doesn't say anything about it getting cancelled out.

Having DMed too much. Nope.

This is easily a rule as intended type of thing. If you hit someone else. Sure fine, this is actually awesome and very creative and totally in the spirit of the game.

If you're just trying to break the game. Nope. RPGs aren't about trying to pull apart the systems in place for a group of friends to have fun. Not at our table anyway.

Even the damage dealt thing is silly. Is argue you cant deal damage to something that is fully damaged.

This is easily a rule as intended type of thing.

Not just that, it's also rules as written, Vampiric Touch explicitly states:

The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.
You can't "hit someone else"* because of the stipulations in contingency. RAI I'm sure they don't want you sucking the life from your left buttcheek to close up the mortal wound in your gut, but based on what they wrote, I haven't found a solid contradiction to this plan in the rules-as-written.

Also you can deal damage to something that is fully damaged. There's even a specific rule for "Damage at 0 Hit Points." under the Death Saving Throws section.
- That still doesn't address vampiric touches life restoration RAI. Heck even RAW the spell says damage to others.
  
  This is still a no go lol.
- I haven’t found a solid contradiction to this plan in the rules-as-written.
  
  If you read Vampiric Touch it's literally in the first sentence:
  
  The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.
*You might actually be able to hit them but the spell at least can't effect them, so no damage, therefore no healing.

As a DM I don't mind rules-lawyering, but you better be prepared for Consequences later on.

I am a big believer in the rule of cool and as long as everybody is having a good time, I don't mind looking the other way if a player makes a creative interpretation of the rules.

...But you better believe I'm gonna spend some time thinking about what should happen thematically if your wizard starts draining their own death energy to revive themselves.

I like a number of the suggestions here. I think my "solution" here would be to let once slide. The second time, I hand the player a folded note informing them that the toes on their left foot didn't come back with them, causing a visible limp, reduced movement speed and -1 to DEX.

...And we can build from there. Possibly start building a Final Destination style plan.

Cheating Death must have Consequences.

Suggestion: Lich Taint

They haven't become a lich by any means, but they've been touched by the aspect of lichdom. Lich's being rather territorial, they will be noticed by any others with associations to a lich, and that lich will be curious...the player will be investigated. If there is a reason for them to remain curious, expect an interview and serious questions.

This is the reason I only DM barebones frameworks like Mini6. No rules lawyeringˆˆ

RAI you definitely would not be able to target yourself. The spell description starts with: "The touch of your shadow-wreathed hand can siphon force from others to heal your wounds." Technically this sentence is flavor only, but in this case it clearly reveals the intention of the designers.

RAW your meme still works, but I would restrict the spell to target only creatures other than yourself for the reason above. Only if the player has a really cool way to describe how to siphon your own life force and somehow convert it into more life force for yourself, I might allow it...

Not a problem, I’ll just grab some of the years from the end of my life and just kinda stuff them into the present to get me through.

Good as new without any downsid- why is my hair falling out?
"The contingent spell takes effect only on you, even if it can normally target others"

I don't think the spell being intended to affect others is a problem here.

I don't think there is any issue with a contingency being on knockout. It seems like a perfectly reasonable and intended trigger.

What is more of an issue is that the spell does require an attack roll. There is no RAW mechanic for allowing a hit, so I think to hit yourself you do have to successfully roll to hit. Knocked out means that a player is unaware of its surroundings, can't move, and is incapacitated. I think even though the spell could otherwise trigger, the character doesn't have the ability to compete the requirements needed at "spell engagement", more on this later. Under this reading a DM could still allow it via rule of cool, but keep it mainly grounded with a con save to allow the attack and/or use disadvantage on the attack roll.

There is another way to read this, as contingency that says the spell is cast ahead of time, but doesn't go into affect until the trigger. Above I had to use the words "spell engagement", which I pulled out of my ass to say when the trigger went off. It could be argued that the attack is actually part of the spell casting. If so then the attack roll on themselves would occur before the knockout and that could let it work. Then the only question is does the damage occur up front or at trigger. I'd argue that if damage occured up front it's a little more balanced, however if the trigger was if something like "immediately before I'm knocked out" (which I personally might require a check at casting for that kind of accuracy) I might let them keep the concentration on the spell as they take no damage and were never truly knocked out. If they take damage at trigger instead, then they were definitely knocked out, and if they were knocked before they were healed the spell went into affect when they couldn't hold it up with concentration as they were out, so the spell is just one time healing, if valid at all as they were never able to concentrate.

Either way, this is not really op rule gaming, despite all the chicanery and hoop jumping. Contingency is sixth level and could work with a normal healing spell if they had one. It's also very likely to not be huge in a fight with the best case being that it was prepared by a very high level wizard who long rested previous to battle. The minimal version requires level 11 wizard who is knocked out and comes back with an average of 10 HP (unless upcasting vampiric touch) and it's not even their turn. The most op version for a level 20 wizard using a ninth level upcast of vampiric touch still only averages 31 HP. They stand a good chance to be hit by a second attack and downed again with the CR of creature they are likely fighting at these levels, and this trick is only really decent once a battle.

Where is the extra HP coming from tho

Vampiric touch counts how much damage you deal, not how much hp the target loses.

It's definitely a flavor fail, but unless you houserule in a cap to damage dealt at 0 hp, (and houserules for how to handle massive damage and the rules for being dealt damage at 0 hp that would be affected,) there's not a limit to how much hp you gain based on the hp of the target.
- What about lore-wise?
The failed death save. Kinda. But that's how I would see and rule it.
I would recommend somewhere non-vital, for instance, left buttcheek.

You take damage that reduces you to 0 Hit Points and fall unconscious
Contingency triggers and Vampiric Touch takes effect
Vampiric Touch is lost beacuse you lose concentration on a spell if you are incapacitated

You trigger after you take damage but before you fall unconscious, like numerous other triggers in the game. I just used the Strength Before Death example because it shows even without magic there's enough time to squeeze a whole ass turn in between those two events.
- You trigger after you take damage but before you fall unconscious, like numerous other triggers in the game.
  
  I fail to see how that's supported by RAW. If damage reduces you to 0 hit points and fails to kill you, you fall unconscious.
  
  The fact that a Fighter subclass gives you an ability to act otherwise doesn't affect this, any more than you could say you stay conscious at 0 because Zealot Barbarians can do it. You don't have that ability. You're unconscious or dead when Vampiric Touch triggers. Vampiric Touch fails.

I’d allow it, but instead of HP damage, you take as many levels of exhaustion.

Thats why you give exhaustion for revival without stabilisation.