Vermaelen wants to leave because he doesn't play at Arsenal. Doesn't make much sense to go to United merely as cover. Personally I don't think either of those you mentioned are good enough, and I don't see what benefit they'd bring other than taking up a place in the squad. Hummels on the other hand would be our only world class defender and could be that for years, with another spot up for grabs beside him and enough cover to deal with injuries.
I understand the logic behind having faith in those we've already got, but I'm struggling to see how signing an average 28 year old centre half helps that. If we want cover, we should sign someone younger with the potential to take over in the future, rather than someone who isn't good enough to start despite being at their peak.