Been playing FO2 for a while now, and I dont understand why I can't get into Vault City. I've completed most quests elsewhere in the game, but I can't get into Vault City. I get told by the entrance guards that I was expelled before, but I havent been there, and there's no second chance. I managed to steal the Vault City entrance Admit one paper from the guard, but I still cant get in. Do I have to load a new game?

Take some drug to reduce your intelligence (it has to be INT 4 or less), and talk with Skeev.

Crap! That won't work if Lynette has banned you.
If you don't have another save, you'll be able to enter the city when you finish the main quest. :/