If you click the join vet games button in queue or on the home page it puts you into an open vet game. The lobby doesn't need to recruit from queue in order to pull from "vet MM". It won't put people in who don't sign up for a vet lobby. A person in the vet lobby has a button to turn it into a regular match on first click, and recruit from queue on second click.
Underdog doesn't need to exist in vet matches but I don't think it does harm. If there's an underdog then there's a significant difference between the MMRs. It's always clear why there's an underdog. It can encourage players to help balance out the lobby.
Ah I see you're talking about an achievement for underdog vet wins. Think of it as a challenge to try to unbalance vet matches.
