Yikes, what a crappy situation and what a bunch of dirtbags owners you have in your league. I would be looking for another league but fwiw here is what I might try to do (imho).
1. Go back and undo the trade and send out an email explaining that it was a mistake on your part. However make clear that the veto rule is in place for trades that other owners deem unfair, not for blocking trades that help both teams. Tell the league you feel there may be some collusion going on and would like clarification from the vetoing owners.
Does the veto rule specify any criteria such as if an owner thinks a trade is unfair they have a right to vote to veto it? Or is it a blank check? If it has specific criteria then maybe you have some options (have them explain their reasoning). If these owners won't/can't explain their reasoning and you feel they are colluding maybe see if the rest of the league agrees.
How many owners voted against the trade?
Basically, try and use whatever rules that are there to your advantage without changing any them. If the rules are sloppily written or non existent then there is nothing you can do.
Lastly, find a new league or lose the dirtball owners (make it clear they go or you do). GL!