Yeah, I'm still not completely convinced, obviously because playing well for Southampton is something different, but maybe I'm focussing on Virgil's questionable matches too much. He was pretty good at Groningen (bet you're still angry Ajax somehow ended up with Van der Hoorn instead of him), but somehow he's never struck me as really, really special, like some people in this thread make him out to be. He's no Bailly.
Smalling has that pace and agility over Virgil, and I wouldn't know if Van Dijk is actually physically stronger than Smalling either, obviously he's a bit better on the ball but I'm not always that impressed with his passing and build up play. And then he would probably cost more than Bailly too, while I'd say Virgil (who was still at Groningen around the age Bailly signed for us) is nowhere near as naturally talented.
I also think, based on a couple of his more unconvincing matches for Holland, where there's a completely different kind of pressure, Virgil would need some time to settle in at a massive club like United. If anything, I think we need to either sign the next Bailly (young and extremely talented), or maybe some top class proven CB around the 28-31 age mark, if we really want proper improvement for our team.