I was wondering the same thing a month ago, so I tested it in SP, and yes you can transform, but it's quite odd, if I remember right it 'flickers' between the two forms from time to time, and you seem to remain a bear most of the time though. I'll retest with hard facts, as my above answer is kinda cloudy in my memory.
I posted about this before the forums were wiped. When you are equipped with full Trang's and in Vampire form, pushing Bear will shift you into human form. This gives you normal cast rate while still having the full Trang bonuses. You will not stay in human form permanently. It does, however, seem to last longer than if you had simply gone through a WP to shift out. If you push Bear while in human form you will shift into the bear. I didn't test enough to see if you shift to vampire only when the bear is supposed to end (based on lycanthropy) or if it is random, but you will eventually shift directly out of bear to vampire after a while.