A vehicle is exactly is as it sounds- a rideable object. They range from jeeps and airboats to chairs.
Few monsters in the Gmod universe have been shown riding around in vehicles usually because they don't need them or they are too slow compared to them to be worth the trouble. Some of the more human-like monsters can be seen using them.
Sniper's van- This is a flying van that drives drives like a normal van except it can fly and park in midair
RED Heavy's truck- A simple old fashioned RED team pickup truck. Can be driven simply by commanding it to move.
Chirstian Brutal Sniper's truck- A rocket powered truck that can also fly