After becoming a vampire through the quest line by having my blood drained in the pool, i'm told to test my powers out by preying on 10 worm assassins. However, if i choose not to prey on said assassins, and instead leave after becoming a vampire, you are reverted back to normal. Which i might add is particularly annoying for anyone low level who wants to become a vampire, as they can't do it unless they defeat the level 40 worm assassins, despite already having the powers, and supposedly becoming a vampire.
Shutting off vampirism to the lower levels is a bad move, we don't want to reach end game then have vampire powers, we want them as soon as possible, so we can enjoy most of the games content with them.