Power armor is armor that walks for you. Literally. It has a power back and synthetic musculature that walks for you, which is why when it powers down it requires enormous strength to even move it.
I would argue that it is not a vehicle because it doesn't move for the Marine of its own volition. The marine walks and it walks with him. Granted, at this point it is just semantics. If you want to use it as a vehicle, then use it as a vehicle, dog. I am just letting you know my opinion to the contrary.