For me, being vulnerable is telling my T things that feel incredibly embarrassing and/or that I'm ashamed of. The things that I'd said that I'd never tell another person or that I don't even admit to myself. Trust and vulnerability have a dialectic relationship: you have to really trust your T to allow yourself to be vulnerable in front of him/her AND if you are vulnerable in front of him/her and he/she reacts supportively, then you trust him/her more. It's really hard getting this tough stuff out. At times I'd sit there for 10 minutes trying to say aloud 10 words. Other times, I'd write them on a piece of paper and hand it to T. But it does help you grow a lot...