No. It is not vetoable in my opinion. The only reason I veto a trade in my league is:
1. There is confirmed collusion (inside trading, trading for a short term benefit and then retrading)
2. The trade is plain ridiculous or is a joke trade (ie. My whole team for 1 guy, ie. Grady for Kerry Wood)
Try not to look at the trade on whether or not you would do it or whether or not you think it's fair (that's for the agreeing managers to decide), but try to look at it as if it makes even remote sense for either or both managers. ie. Maybe the guy trading Grady has a beastly OF and absolutely no pitching. Sure, he could definitely get a better SP than Beckett, but maybe he's got a huge man crush on Beckett and could care less about Grady..who knows.
Just make sure it's not a sketchy trade. Sometimes i'll call the trading managers (usually just the one who is seemingly getting the short end of the deal, just to make sure he intended to. Again, try not to add your influence on the conversation as this may pursuade the manager (who has already agreed to do this deal) to go back on his agreement. You dont want that mess on your hands. Simple question, "did you mean to do this deal?" - Yes or No.
If no - then investigate further and maybe call the other manager
If yes - pass the deal - maybe next time you should be feeding on the weak managers and snagging their Grady's for your Becketts.
Good luck mang.