Full-sugar Coca-Cola Vanilla. Was a bit of a hit in the UK in 2000s, was revived in 2013, then discontinued in favour of their aspartame-filled mess. I tried the "zero sugar" variant and it just tasted like cheap cola with a hint of vanilla. I've since gone for Tim Hortons' French Vanilla since they started expanding here.
In the US, Zevia recently came out with Vanilla Cola and it tastes really awesome to me. It has a bigger vanilla flavor punch than anything Coca-Cola did. I know most people are picky when it comes to sodas especially something that’s flavored with stevia instead of sugar. If you can get over the stevia bit, it’s worth trying. Much better tasting than Vanilla Coke Zero.
Zevia is quite tricky to get in the UK (not seeing any Vanilla Cola imported here) but if I see an opportunity to get it imported, I'll go for it.
Tangentially related, Amazon has a lot of Green Cola, don't know if that's a brand people know or not but they claim to use stevia as well. Edit: Green Cola does contain sucralose so if you're trying to skip that one too, keep that in mind.
What brand, if I can ask? Over in the UK I've only seen Dr. Pepper USA do cherry vanilla, which is different to Coca-Cola's Dr. Pepper that uses aspartame and sugar. (US Dr. Pepper uses HFCS) UK Dr. Pepper was way too strong with the sugar but with the aspartame it just tastes watery and dead.
Just to make sure you're not missing out, are you absolutely certain the kind you tried was vanilla coke zero sugar and not vanilla coke zero? Coke's first attempt at zero was a mess, but the zero sugar line is quite a bit better.