In what sense am I trusting somebody if I allow myself to be vulnerable?
This is the heart of what is required to make a leap of faith, isn’t it? Once you trust, you have comfort because you have placed concern for your safety (or whatever is in question) into the hands of someone else, or some higher power. But what if you deem that no one, not even God is trust worthy because of the suffering you have lived through?
I think the answer is that I am not trusting.
But you are. If you put yourself into the hands of a mass murderer whom you did not trust, would you feel safe?
If I trusted then I wouldn't be vulnerable. Vulnerability implies leaving oneself open to risk. The trust I might place in somebody else would be directly related to my perception of their strength.
Yes
My ability to be vulnerable is directly related to my own strength and therefore directly related to my trust in my ability to care for myself to self-sooth and hold onto myself.
Yes, but it does not follow that just because you are confident in yourself that you would be confident and trusting in someone else.
"How can I best care for myself when in relation to others?"
When there is no trust in others, caring for yourself in relation to others can cause some problems. Caring for yourself WITHOUT relations to others is clean and simple.