If i marry brelyna and turn her into a vampire will she act like a vampire at all and or have any signs shes one. Also how can I convince her to wear different robes? Will she wear necromancer robes?

Well she is a follower/spouse so she will wear what you give her as long as its better then what she is defaulted to wear.

Vampirism... not a clue. I would imagine she would show the effects of being a vampire as any other NPC follower you turned. But again, no idea.