If you're saying that you are vulnerable, isn't that being defensive so in a sense lacking in vulnerability?

No. It's tapping into self-awareness into something (being vulnerable) that is not wanted. So, it actually accentuates the vulnerability.

I don’t think so

Not if you are stating it as a fact or asking for some understanding.
Not a defense, an explanation.
It would become a defense if it were used to manipulate someone to let them get out of something shady or to deflect from their own actions.

Being defensive and vulnerable aren't mutually exclusive

Being defensive is not the same as being defended. Defensiveness is an attempt at defending. Not necessarily success at doing so.

No.