Starglim |
A vampire's Change Shape ability emulates beast shape II, which is a polymorph effect.
The transformed vampire loses its extraordinary and supernatural abilities, natural attacks and movement types that depend on its original form. Its natural armor bonus is not any of these, but I believe most GMs would say that it's an ability dependent on form. Note that the wolf form has natural armor of +2 from transforming into a Medium creature using beast shape.
I don't think energy drain depends on form, so the wolf form would retain it.