Not sure if you have to be online, but go to either Hyperious or Master Gee and touch (run into) their gates. This should trick your game into thinking there are 4 players (it's easier for Verm to spawn with 4 players)
To check to see if this has occurred, open an ammo crate and try to to pick up bullets for a weapon you're maxxed out on. If it does, your in "4-player mode" --- You can take my Karma, but you'll never take......MY FREEDOM!
Actually funny you should ask- yesterday GBX servers sent out a hotfix that inadvertently had vermi spawning at record proportions. It's all over the forum. The 4 player slider works- but it makes the game quite difficult. GBX said they fixed it- but I still spawned vermi six times today. I dont think its fixed, but the flip side is vermi is feast or famine with loot- one time he dropped two white Dahl pistols. TWO!
Yea yesterday one appeared out of nowhere in the Caustic Caverns as I was playing split screen with my kid we made a right after coming out of the minecraft area and there it was. I to my kid who is 9 let's kill it and he said no and fled leaving me . I fought really hard and died but did enough damage that when I returned to the area after respawning the foul beast was dead too and unfortunately he didn't drop a damn thing. --- GT: REdrUM0351