Hi there,
The changes from vanilla Skyrim to Dawnguard are as follows for vampires:
You no longer get attacked at Stage 4
Your fire weakness and frost resistances have been changed from 25%, 50%, 75%, and 100%, at stage 1, 2, 3, and 4 respectively to 20%, 30%, 40%, and 50%.
You get glowing yellow/orange eyes.
You can now use Vampire Seduction to feed on the people you've used it on.
That's all the changes I know of. Feeding should still revert you to stage one.