If you follow the logic that an already-cast-and-concentrated-VT can't be used to make opportunity attacks, then you run into a weird scenario if the Wizard does happen to have the Warcaster feat.
Because then you get the following:
* He still can't make an OA with his VT, because nothing changed in the rules.
* However, he can now stop concentrating on the old VT, cast an entirely new one, and use that to make the OA.
To me, that is insane. The idea that a feat allows you to perform the second while still not being able to pull off the first is counter-intuitive to the point of sounding like a Murphy's Rules parody. Like a bug in the programming it causes silly behavior.
Look, guys, this is 5e. The writing is fast and loose and "natural" so there's little point in analyzing the precise wording of spells with added emphasis in bold and italics. This is not 3e or 4e, that kind of approach just doesn't work all that often. The RAW is so vague on this issue as to be essentially be silent. But if I think in practical terms, that the Wizard is basically only reaching out and touching the orc with his creepy hand, then I see no reason whatsoever not to allow the OA.