There aren't any tanks or heavily armed vehicles, but you can find technicals (small flatbed trucks with a machinegun mounted in the back). There are also a couple of helicopters that are armed with machine guns and rockets. Ammunition is limited and vehicles cannot be rearmed (as of 0.1.34).
To fix a vehicle, you will need a toolbox. Toolboxes have a limited number of uses before they're used up. If it's a land vehicle and there are wheels missing, you'll also need tires - one for each wheel. It can be difficult to carry enough tires to fix a large vehicle.You'll also need fuel. You can find jerry cans with which you can carry fuel. Jerry cans can be found full or with some fuel in them. If you have a hose, you can siphon fuel from other vehicles and fuel tanks that you can find at various locations. Fuel can also be found in the most obvious places - petrol stations.
Some of the enemy forces are in possession of vehicles. These are mainly technicals, but there is a helicopter patrolling the skies.
You can store items inside a vehicle, but there is a limit.