can i have acces to base/buy base if im vault city citizen ?
Actually yes you can, if some high ranking member of the base adds you as a friend in the terminal database. Once it's the done, the next time you enter the base you will receive a message that looks like this : "You now know the way to this location". This way you get access to base without actually joining the faction !
You will lose location when you die in the base, so dont do anything stupid.