VT is actually harder for me than Vicious sometimes. I guess there's less of a gimmick and there's a known wiki strategy for VT runs generally. Vicious dungeons also let you plan your preps toward the boss, and a lot of VT runs don't.
The other issue with rainbows is that rainbowing a dungeon takes a long time. What is it now, 21 wins minimum? If you don't start on the badges early, you might need a few extras as well.