Does experience earned vary when you kill enemies from a vehicle?
Solution 1:
Yes, from experience, you earn less XP from killing enemies in a vehicle. It's orders of magnitude different.
You can damage an enemy from a vehicle and kill them on foot and get the "non-vehicle" XP for the kill. I've done this before in co-op, when I would hop out of a vehicle and my partner would stay on the turret, dealing damage. The game appears to only care about the killing blow.