Ah, true. Wouldn't vile damage be converted to nonlethal damage by regeneration like everything else, though?
Possibly, but it'd still be vile, and so unhealable outside the conditions of healing vile damage.
Vile damage, like regular damage, results in the loss of hit
points or ability score points. Unlike regular damage, vile
damage can only be healed by magic cast within the area of
a consecrate or hallow spell.
And when their vile nonlethal damage exceeds their HP, they're permanently unconscious - plenty of time to go fetch a Regen-beating weapon and CDG them.
This isn't based on a specific first-party ruling (Wizards never addressed the issue at all, actually) - but every source I can find, include the professional cheesemongers at Brilliant Gameologists, agree that at best, it becomes Vile Nonlethal.
if your still accepting chacter would a half fiend/half dragon work?
A half-dragon fiend? A half-fiend dragon? A half-fiend half-dragon something else? These questions are relevant here.