has Vaughn ever bottomed? Would he ever bottom in the future?

Oh no, Vaughn has bottomed! In fact - and this’ll come up later in the series, but it’s not really a spoiler - he’s bottomed for Allison Song more than once.

Vaughn doesn’t consider bottoming to be an inherently submissive act. But even if he did, if he decided he wanted to do it, he would consider himself the exception to the rule. Vaughn believes he’s superior to all other people, and is literally incapable of interpreting any of his own actions as submissive.