The whole key with vocation leveling if you're not all that worried about level leveling at the same time is to have the battles be quick. Therefore, the earliest places in the game that have level 99 caps are the best places for it because those battles are generally easiest to just dominate and win in one round with a high-level team.
Most anywhere under the ocean works for this purpose, as does the Spiegelspire, as does the Mt Snowhere area as does the cave south of Alltrades in the Dreamworld, as do a few other places. You have many choices. --- Frazee uses the lunch!