Ok, I too ran into problems with vehicles.

You can do one of several things:

1) Try having your other toons sit in vehicle too... some (like in battlegrounds) allow multiple riders

2) if not, try just mounting your MAIN in the vehicle and have your other toons follow. I was able to do that with "fresh remounts" and with the plague spreader quest ... other toons got credit cuz they were close... didn't have to do individually.

3) move em all individually near where you need to be and pull things close enough to get credit ... I did that on a flying quest in wyrmrest... I mounted the dragon with other toons sitting on ledge, and completed the kills by pulling the drakes near enough for them to get credit. then all I had to do was dash off to the final objective on each toon in succession.

4) Just do it N times where N= number of toons... I actually REALLY enjoyed the first tank one (the one with the horde tank by the misty harbor) and liked running it 3 times.