Given his age, there is a non-zero chance that he may die while in the presidency?
JD Vance isn’t exactly a charismatic individual (some might even go so far as call him ‘weird’); so this entire neo-fascist movement may very well fragment and flounder in the not too distant future.
No doubt he’s smart, he hitched his wagon to the Trump train after all - but I don’t think that’s particularly important.
He doesn’t have Trump’s charisma, so he’d struggle to maintain control over his fan base and would likely face challenges from key faces in the cohort (eg. Ron DeSantis, MTG etc.)
This is basically my reasoning as well, he will most likely be eaten alive by the true believers and power brokers will break him over knee. All of Vance's influence is derived from Trump. And especially if Trunp dies before inauguration he may very well become irrelevant depending on certain factors.
If he dies before they are able to really get the structural changes in P2025 off the ground, Trump's death will likely instigate the death of the movement. If it dies after that the P2025 restructuring has happened, won't matter.