In 4th edition, Player Characters are distinctly different from Monsters (which include NPCs). The core rules and balance behind both PCs and Monsters is inherently incompatible.
For the example you've given, I'd have the player change their class to the vampire class (accessible via D&D Insider Dragon magazine and Character Builder Tool) or as you say allow some feat changes to show the extreme differences. Also I agree with Oblivious Sage that its actually a terrible class and I also think the vryloka (sorta vampire race) is a terrible race choice as well. Why not make him being a vampire be something that affects him from a story perspective, but he is still the same race and class combo. There may even be magical items or alternate rewards (like boons) that replicate vampriric qualities.