Not sure, but I doubt you can join VC after you left VC faction once.
afaik you cant go inside the vault unless you are a citizen so yeah plan busted.
Yeah, you need to be a citizen to get in the vault. Also, play a different character if you want your rep to get better. It won't if you're logged in.And lastly, I had a buddy with the same issue, trying to join VC twice. I asked a GM, and it is possible, but tricky. You are only presented with each option to join once, so you can join two or three times. However, you have to do it in order. So if you bypass the quest option (get the hy-mag, which I think is the first way) and get in with your skills, then you can't rejoin. But if you get the hy-mag, leave, then later get in with a high FA skill or whatever, then you can join twice.
There is a day pass, so yeah myth busted...