how do i get the last vehicle?

To ahcieve the last vehicle (snowmobile) you must complete all of the single player missions with a status of "great"

Its called a hover sled but it does work good as a jetski and a snowmobile.

tompie's right. The Hover Sleb is possibly the best vehicle in the entire game. It's untouchable on ice and water. It's speed is very good. The climbing ability on this baby is better then even the Russian made Kovastov. It has arguably the best counter measure in the entire game. The only downside to this is it's handling requires some skill; the Hover Sled can't make sharp turns because it has no traction.

If you have questions about getting a great in mission, feel free to ask me or post it somewhere.