Well, since the issue doesn't seem to be what defines a vetoable trade, but rather how we should determine if a trade is vetoable, how about a poll?
Vote for either
1) Commish/Co-commish ruling based on strict definition of collusion
2) All league owners vote
If we go with option 2, we would need to decide if we only need a majority vote, or 2/3 majority, whether or not those involved should be allowed to vote, and what to do in case of a tie vote (even number will be voting).