I think the ultimate thing that's required in order to feel vulnerable is that you allow yourself to love/care about the other person, so that they are in a position where they could reject and hurt you. The most vulnerable times for me are when I truly care about someone, but show them my true self as opposed to being too afraid to open up, or trying to look better etc. Like when you are open and raw and exposed to someone whose opinion you genuinely care about. In my mind you should be careful about letting yourself become vulnerable, I like to vet people out before giving them secrets that could harm me because there are bad people out there! Almost all of us on here I'm sure have been burned badly in the past, so it is no shock we would struggle to allow ourselves to become vulnerable. My take on therapy was that if you can do this with your T, it kind of mimics a parent child relationship feeling kind of and can be very therapeutic assuming your T makes sure things stay safe and is competent.