Let's do some analysis to give V a river range. He didn't 3bet preflop and you have both K and Q blockers so he probably doesn't have AK/AQ. AJ w/ J
is 3 combos. There are 6 combos of sets. AT, A8s, and T8s make up about 12 combos. Flopped flushes are QJss, Q9ss, J9ss, 97ss, 76ss, 75ss, 65ss, 54ss. However, maybe he's tight enough to fold some of the smaller ones pre or lag enough to 3bet them pre. Plus, there's a decent chance flopped flushes raise flop for value and protection since the cbet sizing is small and its 4 way on the flop. So this discounts combos of flushes he gets to river with down to 3-4.
So the vast majority of his river range will be sets and 2 pair. This means if we want to bluff here we want our story to say "Hey, your set/2 pair is no good because we have the flush." The best way to sell this story is with an overbet for some amount that puts those hands in a pukeworthy spot. I think that amount would be $350.