So you're saying that working vehicles should be as uncommon as... power armor?
Frankly I don't see what is so 'unrealistic' about it. I don't imagine that when one mentions working vehicles in Fallout he refers to highways with traffic jams! If there are hundreds of working computers, robots, vertibirds (they are literally vehicles aren't they?) and at least one car (in FO2) why are a few more so out of place? Perhaps there will be 4 or five working cars or motorcycles and the player will have access to one of them. Maybe they could be even built out of parts of other kinds of working machines and powered by other means - maybe steam could work or coal or even solar energy or whatever...
Come on... even bicycles are vehicles and don't tell me it would be impossible to build one in a post apocalyptic world or build a motor of some kind and attach it on it!
Not impossible, improbable. Those vertibirds are military, operated by a military organization, with, one assumes, stockpiles of parts, or other vertibirsds to cannibalize for parts. That car in FO2 didn't act as a car, but rather as a mobile locker One didn't DRIVE it anywhere.
Again, as pointed out, the use of any kind of vehicle requires physical infastructure....roads, paths, tracks for trains... and the lowe3r the quality of this infrastructure, the less likely that the vehicle will be useful or durable. The road ssytem in FO3 strikes me a s a accurate representation of what roads would be like. No car could operate on them, and although brahman may traverse part of it, there are places where even they couldn't go. I was reading a history of cartage in England during the Victorian age (Yes, I'm a history geek) and even then there were many places where horse drawn wagons could not go because of lack of proper roads and trails. In the move West here in the US in the 1800s, there were only a few TRAILS one could use for wagon trains, due to natural and man made limitations.
So, horses or pack animals I might be able to see, if you can take enough food for them with you, but much of anything else seems nonviable in terms of setting. And, as pointed out above in an excellent post, in terms of game design, the faster the transportation, the larger the game world needs to be, and since we are talking about a Beth style fully rendered gameworld, that seems impossible. If we are talkinga bout a overland map, then no real transportation is required, but something like the car in fO2 would suffice as a prop.