To what degree does this hold. Yes, in certain cases showing vulnerability is endearing.

But imagine an anxious person that obviously struggles to talk to you, that forces herself to interact even though she's not very good at interactions. In this case the vulnerability isn't endearing at all. I've witnessed people in such situations and the result was non-edearing.

On the other hand, I think of a friend that has high status but always makes himself vulnerable, giggles and blushes. In his case vulnerability causes endearment.

I'm not claiming to know which essential feature separates the 2 cases. I am however explaining why the general 'vulnerability endears' is context specific. I does not seem to be the vulnerability, at least on its own in isolation, that causes the endorsement.