Definitely not veto material. As Payne Dailey said, you have to let owners manage their own teams. It's not your job to be Big Brother and look over everyone's shoulder to make sure everyone is doing what you subjectively think is the best for them.
Some reasons why one might prefer LenDale White over Ronnie Brown:
-Tennessee's offense looks to be much more dynamic than Miami's.
-Vince Young is a better quarterback than Trent Green and will open up things for the RB more.
-Ronnie Brown may have already proven to be mediocre. LenDale White hasn't proven that yet.
-Miami may be playing from behind more than Tennessee, and so Miami may have fewer total carries for RBs than Tennessee.
-IIRC Miami has a pretty tough run schedule (this is just from memory); Tennessee may have an easier one.
-Jesse Chatman could steal carries from Ronnie Brown. One possibility is that he would steal goal-line carries, so that Brown is not injured again. LenDale White will presumably have goal-line duty in Tennessee rather than Chris Brown.
-LenDale White may have a higher ceiling than Ronnie Brown.
Obviously these are all debatable - that's not the point. Most things are debatable.
Bottom line here: groupthink is unfortunately much too big of a factor in fantasy football; people feel compelled to go with "common knowledge" in terms of player valuation; just because someone runs contrary to the common perception does not mean that he is a bad manager; everyone is entitled to his own way of ranking and valuing players; it is not the commissioner's job to determine how each manager must rank and value players.
If this trade were vetoed in my league, I would likely immediately seek a replacement for my own team.
When I read "trades may be vetoed if they are very lopsided" in a league's rules, I presume it to mean that something like the following trade would be vetoed:
Team A Receives:
Team B Receives: