PLEASE someone correct me if I am wrong and this has been changed in an update but my understanding is that if you are in a group while doing this and you choose to divide and conquer the pre-vaults, you will be ported to your party member if they are engaged in the last room of their pre-vault. This causes all sorts of issues.
-I do NOT share your feeling that this is a group activity.
-In your defense though, having to do all these pre-vaults before the actual thing is so flipping annoying that I don’t know why they didn’t change this in the last “emergency-oops-we-don’t-listen-to-players-patch”.