I'll start by saying that working vehicles are incredibly rare in the zone, and generally any working vehicles that are in the zone will have long since been claimed by one of the factions or imported specifically for a faction, so as a general rule, unless your character didn't mind both finding a building where, say, a UAZ is being stored and also killing the numerous guards who will be keeping it safe, you're not going to find one. As for how fragile they are, they won't blow up from a single bullet hit, but if one happens to get lodged in the engine block, odds are your working vehicle will soon not be working. It probably won't explode, but it will definitely stall in not too long. As cool as Hollywood style explosions are, shooting a gas tank with anything less than a high explosive or incendiary round will only make it leak.