I was at level 20 with a Sith Marauder on Harbinger and entered a War Zone while Vette was active. When I left the War Zone, her icon was gone from the main screen, though she still appeared on the companion screen under N. When I tried to hit the button to bring her back, no response. It was like she didn't exist; it didn't even ask if I wanted her to sell my generic items. Luckily, I had Treek, so I made Treek my active companion and, once she appeared, tried to bring Vette back using the hot button on the N screen. This time it worked. A guild mate of mine had the same problem on a Juggernaut, and couldn't get Vette back until he got his ship and could make the ship droid his active companion. He said that she was active in conversations, but not visible in the cut scenes.