For your best chance of getting Verm to spawn be on UVHM and have 4 players. He does not spawn on Normal.
If you dont have 4 players but have the Pirate's Booty DLC you can use the 4player scaling trick. All players in the game need to touch (not even open) either the Hyperius' Arena gate or the Gee's Arena gate. Thats it. The game is then scaled to 4 players and you have a much better chance of getting a Varkid to evolve all the way to Vermivorous.