I am the commissioner in my league, but as of this year, I allow the teams to vote, which so far has been fine. I am not sure how to rate this trade though.
Teixeira and Ethier are both elite hitters. Of course, Tex is a bit better....but Ethier isn't that far behind.
Berkman is an average, but definitely servicable, fantasy hitter. Wagner is an average, but definitely servicable, closer. Since I value hitters more than I value pitchers (especially relief pitchers), I would rate Berkman above Wagner. But they are both in the same ballpark.
So I see this as a very reasonable trade. Each manager is trading one elite and one average player. You could argue which side has more pure value....but at this point in the season....needs are at least as important as the pure value of the players. And, depending on needs of each team, I can easily see this as a potential win-win trade.
In a vaccum, I would prefer the side getting Tex and Wagner. Tex is the best player in the trade and closers are an obvious value. However, if I was really struggling in the hiting categories...and I had enough closers to spare Wagner......I would prefer the combination of Ethier and Berkman, which is going to produce signifiacntly more total stats than Tex will.
So unless there is something you aren't telling us and this is a situaltion where the trade makes no sense at all based on the needs of the two teams (i.e an experineced player is talking a newbie into punting saves mid-season)....there is no way it should be vetoed. If managers are voting against it you need to get them in line. Tell them that just because they wouldn't accept the trade, that doesn't mean they should veto it.
As commissioner, you should know that this is as fair as fair gets. Why would you question this? Maybe we would better understand your reasoning after hearing back from you. This is so fair...
P.S. Is this a league filled with people you know and is a group from previous seasons? If so, there shouldnt be a Veto on any trade unless its just so obvious that the dude is tanking on the league. If its not, and there are a lot of new guys playing in a one yr. league, sometimes you do have to watch for the obvious, I don't care manager.