Having too many vehicles will destroy fo IMO. I really hope they would only have major factions have a few. You getting a hold of a vehicle except on 1 special mission would be a disaster IMO.
Indeed.
What I imagine is a game that involves Motor City. There is an old Chryslus factory, which is in the possession of a huge group of raiders (friendly raiders, like the Khans, that you can joind and do quests for). These raiders are all mechanic freaks and has lived in the factory for decades. They scavange their surroundings for car parts (which you can help with), their factory is full of half-finished cars and all. They even build their own kinds of vehicles and machines. They go out raiding with these vehicles aswell, but they are not merciless in their techniques. With this faction you can earn a car, and they can upgrade alot of you.
Then of course there are a few NPC's scattered through the world (which should be much bigger than F3 and FNV) that maybe owns a vehicle or a broken vehicle that needs repair or just knows how to repair or mod a vehicle for you. Paint jobs, even.
Then this faction is also involved in the quest somehow. There's maybe two major powers in the area, and both would like vehicles. They've probably both got a few of their own pre-war vehicles working with the help of their own mechanics, like pre-war jeeps and trucks (I do think there are a bunch of working vehicles in FNV for example, but the engine restrict so much that they just have to stay put. Like the trains for example, and those trucks in different places with NCR supplies etc) but they don't have the knowledge, resources or means of production like this faction has. This faction also has something of greater importance, maybe?
Then there is also this more evil raider gang that rivals the motor faction. They are much more merciless, they attack tribes and kill women and children. The motor faction has more of the attitude of the pirates aboard http://www.imdb.com/title/tt0303461/. This evil faction has fought the motor faction a couple of times, managed to steal a few lighter vehicles and motorcycles. So they use them on some raiding parties.
Of course I'd like many different kinds of vehicles so there'd be a vareity and all, but fans would complain if they only could drive one kind and I wouldn't like it to be all GTA over this so... vehicles would be somewhat rare. There is this one faction that has and produces a bunch, then there's this raider band that has a few, then a couple of spread-out NPC's that has some backstory and quest related to them that maybe owns some kind of vehicle. Then two major powers that has a few...
So the thing here is to make you not being able to obtain a vehicle that easy. So, to obtain one you have to shoot at it, kill its driver. That might damage the car so that it won't be pleasant to drive, or it was already in a bad state to begin with (like weapon and armor degrading). Then, if you find one parked, it might be locked. Of course you can lockpick the lock and hotwire it or something... but since there are few vehicles, people will recognize stolen ones. Factions most likely put labels on them, or they are unique in appearance. One of the major factions, as I said, might just have pre-war military vehicles and driving around in one of those will get alot of bad attention. And if you have one from the motor faction, it probably has some insignia or a unique design. The few unique NPC's that has a vehicle most likely has a unique one aswell, and they might be harder to obtain aswell like really hard locks, traps or guard dogs.
Or the easy way - you can't hotwire cars. You need to obtain the car keys. You get them by doing series of quests or something, doing a great favor, or buying/trading for the car (or kill the owner, which might also attract bad attention to you and your car).