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.

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.