IMO vulnerability has everything to do with trusting ourselves, and very little to do with other people.

If you trust someone completely (does this even exist?), then you're not really being vulnerable when sharing or being with them, you're just playing in that trust.

I think you build trust by being vulnerable, and intrinsic in that is the knowledge that you will likely be hurt, disappointed or unheard.

You've got to be able to trust yourself and your ability to handle it when those inevitable hurts occur.

That trust in yourself opens the door to trusting others.