Best way of doing this is to play the Combined Arms Moshpit playlist that's under Featured. The best maps are Armada and Crossroads. Use the boat on Armada and tank on Crossroads. To keep track of how many vehicles kills you've got, got to Barracks -> Medals ->look for the medal called Weapons Hot.
The tank is a bit easier I think as you can stay back and snipe people with it. The boat is harder as you're more exposed to C4 and launchers that you can use from under water.