The issue is going to be whether we can find a role for him within the system.
I did an article on him about 3 weeks back and in fairness he has been very impressive. However what we should have learned over the last 2 years is that collected individually good players and lumping them together rarely works out well.
For CSKA he is player as one of 2 number 10's (in a 3-4-3). I can't imagine us playing that way, or ever playing 2 number 10's given Silva's desire for wingers. In a central role, I think Silva wants a very mobile 10 who can get past the striker and help the press high up the pitch. Cleverley did this very effectively at Watford. It's difficult to see Vlasic in that role or even as one of the 2 deeper lying players.
I hope the kid does really well. I suspect he will go on and have a very good career. His performances so far give us every opportunity of doubling or even trebling our initial investment (of 8-11 million) and thats fair enough for me. If you bring younger players in, improve them and move them on at a profit you are not doing a lot wrong.
Alternatively though, Silva may well insist on integrating him and see a role. In fairness to the club they backed him over Lookman in the summer, so can't see him being sold unless the coach wants him gone, which is how it should be.