VS has engaged in talks about joining *A some years ago. I understand UA made it very clear that it would veto it (presumably out of fears that VS would cannibalise UA's premium business between the US and London).
It would only be in VS's interests to join *A now and it has shown recent interest in doing so. It simply suffers from not being able to take advantage of much transit traffic at its hub at LHR as a result. Of course VS now codeshares with UA because UA inherited the CO codeshares, so who knows what the UA view of it is now?