If you go to the DPS section, I have a red mage thread there and I explained that.
My LS and I grouped up and we accidentally Q'd up for the Vault specifically without having a healer in the party. And we're like "we can't vote abandon for 10 minutes, so let's see how far we can go."
We wound up getting clear to the last boss with nothing but Vercure for healing (the two DPS died at the very end of the 2nd boss but we still killed him). We made 3 attempts at him and one attempt we got him down to 15% but we just couldn't do it. There's just too much damage flying around and he kept pulling cheap combos.