My problem with Vehicles is a motorcycle would not allow you to sneak up on anyone, the distances involved are not that far like in FO1/2 between cities, and in the real Fallout World, the darn thing would get stolen all the time.
It's like the Horses in Oblivion, seemed like a good idea at first blush, but the gameplay was too gimped by using them. As you had to dismount to interact, fight, take loot, etc etc.
yep the engine svcks too much for vehicles to be any fun.
and onto the car thing - i believe the reason none are working is because the components needed to start the car and keep it running are well over 200 years old in most cases. the nuclear reactor is still going, and is infact quite dangerous.
in FO2 you have to get a fuel cell controller, but you also have to find a highwayman in acceptable condition. i'm sure the issue is that the electronics are all shot and require a massive amount of work that simply can't be done in the current FO world... microchip manufacturing, etc. even modern cars probably couldn't be replicated in the fallout world though.
but a good old mechanical-based engine would do the trick... if you could find a car from the 60s and 70s ;P.. just gotta make it run off something besides petrol.
then again, the brotherhood in tactics had a wide supply of hummers they supposedly got running themselves. so maybe in the next fallout we'll come across a warehouse full of operable cars or something. i imagine that if you could find such a facility that the cars would still be functional, even after so long, especially given that they are nuclear powered.