Oblivion had horses. I don't know how different could that be but they could've used some of the code from that for vehicles.
I don't see why a DLC with drivable vehicles would be impossible. If nothing else, they can only be driven on certain areas, but it can be done.
People seem to forget that those horses wre absolutely terrible. They moved awkwardly, looked bad and you could easily outrun one on foot, using a horse was completely pointless in most cases. Add to that that mounted combat wasn't an option, and horses ended up more annoying than practical.
Sure, there were mods that made horses better, but not by much. There was a game called GUN, it was one of the launch titles for the 360. That game had far, far better horses than Oblivion did. Why? Because it was designed with horses in mind, Oblivion was obviously not and if it was, then it was simply poorly executed.
There's really no excuse for Bethesda to have inferior mechanics compared to earlier games, Red Dead Redemption did horses well, some say the controls were bad but I never noticed that, once I knew how they worked it was easy as pie to outride anyone and shooting from horseback was a very viable combat option, at least at long ranges. If Rockstar can do horses well, Bethesda can as well, the technology is there.
As it stands now though, no, vehicles are not feasible. There are some that work, but they work badly. I'm not saying the mods are badly made, they're not, they're doing what they can with what they have to work with, and that's not much.