Could an efficient vehicle be designed that only uses high powered magnets for its' motion?

IF "high powered magnets" means permanent magnets, then the answer is NO, that is a perpetual motion machine and flat out impossible.

IF "high powered magnets" means electromagnets, they yes, that is an electric motor, and there are millions of vehicles that are powered by electric motors, such as diesel electric locomotives.

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In theory yes.

The only problem is the weight of the magnets...and also getting them moving in the first place.

And if you wanted really high powered light magnets you'd have to have electromagnets (Which would defeat the whole point).

You're better off sticking to electric motors or hydrogen fuel cells if you want greener energy.

Im sure if magnets could be used to power cars then it would have been done long ago.

You do not specify the type of vehicle.

Go to Japan and ride on their bullet train, that uses magnets for its propulsion and it goes at quite a lick, Over the "ton" and it pulls a long line of carriages.

Yes - the Japanese built a boat that used magnets and seawater. It had no moving parts.

Nope.

Yes, they are already being used.

http://en.wikipedia.org/wiki/Maglev_train