Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
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.
Where is the extra HP coming from tho
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?
Seems like you can target yourself but it can only heals "From others".
I would recommend somewhere non-vital, for instance, left buttcheek.
The failed death save. Kinda. But that's how I would see and rule it.