It would seem to me that Barnett and Overeem are taking some pretty regular tests as well as GSP currently doing the VADA yesting... What testing is Vitor going through ? (Not arguing the quote just curious on your source of this)
But the fact is, Belfort is probably the most tested TRT approved fighter in the UFC. Being the most regulated user, Vitor is definitely not abusing TRT as he would never get away with it at this point.
A few you mentioned here were not succcessfull because there motivation/mentality was not there anymore. You can;t say both Sonnen and Henderson have been failures while on TRT they've both had some good runs of wins at various points. In Dan's case TRT doesn't stop you from falling in love with the overhand right and thinking you can't be knocked out
TRT was the only reason for Vitor's recent success, how would people explain away other TRT users failures to go on a significant run? Forrest Griffin, Rampage Jackson, Frank Mir, Nate Marquardt, Chael Sonnen, Dan Henderson and the list goes on.