After reviewing this thread, I have noticed one vehicle was forgotten; The Mothership. It's the hippie van that The Truth drives around in. I don't know how to classify the vehicle (rare or unlockable). The only time I have been able to acquire this van is at the beginning of the mission where CJ and Sweet drive back into Grove St. at the start of the riot. Simply get out of Sweet's car jack the van and drive it to your safehouse right up the street (provided you don't have another vehicle there).