I don't know if this has ever been specifically addressed in a thread before, so I thought I'd go ahead and start it. What I mean by vulnerability has a big emotional aspect, but what I want to point out is how it is really not so separate from our thought process.
Often times we find that people who disagree will do so out of 'emotional reasons', but I suggest that the emotions are secondary and is really a form of defensiveness. In fact, if you pay attention to the whole process, you may notice a certain visceral feeling where it feels like a contraction, and then the emotional response either distances, blocks or turns away from the issue at hand, and the one feeling threatened will stop responding to reason.
We might consider this as a recognition that the mental identification is threatened, which is to say my religion, my way of thinking, experiences and the like. So in that sense for someone to be really open they really are in a position of vulnerability, and that vulnerability is there in so far as they identify with whatever that might come under question. (which is to say it is a subjective experience, ultimately dependent on their frame of mind)
So in this regard I think we would want to be careful in how much we choose to pursue certain issues which may be key and where the person might feel especially threatened. If someone is going to open, it seems they really need to feel safe enough to be that vulnerable, otherwise there's just going to be a reinforcement of that defense, which amounts to blocking things out of consciousness.
So this isn't really a new point, but hopefully it is useful to kind of break it down and go into it with a little more detail - and bring it into consciousness.