Your characters only look how you make them, they don't change when they become infected, poisoned or beaten up etc
Really? Even on Stage 4 Vampirism? That svcks...
How does the population of Skyrim know to attack you on sight if you look like a normal person?
EDIT:
Wut? Did you even read the OP? Vampirism does change the character's face.
OP try going a few days without feeding; otherwise it's probably a bug.
Ah, phew