I am playing a Doppelganger, and I would like to become a vampire. First, is it possible for a doppelganger to become one? Second, what exactly changes? I read that one loses all racial abilities. Does that mean i can no longer change shape and detect thoughts?
“Vampire” is an acquired template that can be added to any humanoid or monstrous humanoid creature (referred to hereafter as the base creature).
Doppelgangers are monstrous humanoids, so they are eligible to become vampires, yes.
A vampire uses all the base creature’s statistics and special abilities except as noted here.
So no, a doppelganger does not lose any statistics or special abilities except as noted in the vampire entry.
Increase from the base creature as follows: Str +6, Dex +4, Int +2, Wis +2, Cha +4. As an undead creature, a vampire has no Constitution score.
Your doppelganger would lose his Constitution score, going to Con — and thus using a +0 bonus on anything he adds his Constitution modifier to. This may result in less HP, almost-certainly results in lower Fortitude saves and Concentration checks, and so on.
For the rest of the changes, please see the vampire entry. The changes are just applied on top of your doppelganger.