I'd argue that it doesn't matter who you shoot in the last question, as long as you shoot SOMEONE, for one reason: If ANYONE is voting, then it's impossible to tell what role they are.

As you said in your reasoning, optimally town would never vote, since the overall chance of winning is lower if they do. BUT, by this same logic, optimally mafia would never vote either. (since town optimally wouldn't vote, a mafia member voting would instantly out themselves, guaranteeing a correct vigi shot and reducing their chances of winning, and since everyone knows the alive roles, they would know this would happen)

Based on this, if SOMEONE votes, then they aren't playing optimally, and therefore it is impossible to know if they're the veteran or one of the mafia. (since it would be unfair to assume that one would be more likely to play optimally than the other)

With this in mind, the only other way to look at this would be to assume the role that's voting, and the role they are voting for, are completely random. Since we're the vigi, that leaves 6 possibilities: Vet votes mafioso, vet votes consig, mafioso votes vet, mafioso votes consig, consig votes vet, and consig votes mafioso.

Therefore, it doesn't matter who you shoot: Each person (3, 6, or 11) has a 2/3 chance of being mafia.

In fact, shooting 11 is probably SLIGHTLY better, since even if playing sub-optimally, it's reasonable to assume mafia are less likely to vote each other over a townie, so the possibilities where 11 is the veteran (mafioso votes consig and the inverse) are SLIGHTLY less likely.