Given your description, it seems like V is a TAG who is capable of identifying decent spots to make moves for small amounts, but gets MUBSY when the amounts get larger (checking behind 75s).
When a 'good TAG' opens to 4x from MP over a limp, this is not going to be the sort of spot which a MUBSY V will consider good to make a move in. Additionally, he 3-bet AKo fairly small earlier, which usually allows us to discount that from his range. He's also OOP here, making him even less likely to be 3-betting us light. Against a tight player's value 3-betting range from the SB, we can just fold pre.
As played, calling flop is marginal, but we have to ship turn since pretty much all of his range has at least 15% equity, and that's even without a diamond in his hand. There is also a non-zero chance that V will fold AA or KK without a
to a turn shove here (although this is LLSNL, so it's still a fairly low chance without history). Even when called we have between 6 and 10 outs.
As played, on the river a call is OK, since after he checked behind the flush on the paired board, his value range here should pretty much only be QQ for quads, with a chance of TT (unlikely to 3B pre). From Vs sizing on both the flop and river he doesn't seem to want to commit too much of his stack (despite already being committed), and he doesn't bet thinly for value, so this makes it even more likely he is bluffing (with AK, probably with a diamond.)