http://archive.wizards.com/default.a...d/sp/20030824a
As specified here, the vampire template class can be stopped mid-template to take a different class.
Characters are not required to complete all the levels of a given template class in uninterrupted succession. For example, a character who takes a level of wereboar could then take a level of fighter and a level of rogue (or any other combination of other class levels) before taking another level of wereboar. A character must still take the first level of wereboar before taking the second, just as with a normal class.