I also need to do that quest. I can meet you today outside the vault and maybe we can get the quest done together.
Or at least, we should have at least four people unless you have a high level. I am a level 49 Ice Wizard and there are at least 8 floors. I made it to level 7 before I ran out of life and potions.
I have been trying to do the vault with 3 wizards, but they keep dying and I do too but I have all of my potions up and I am a level 43 Ice wizard!!! HELP ME !!!!!!!!!
Don't go rushing off into the next fight. Setup your defenses and attacks according to the monster you will be facing. It's a long dungeon, but not impossible.