In the game vehicles are not only considered tanks, boats, jeeps, helicopters, and armored cars but are also considered all types of turrets.
There are many ways to kill a vehicle, if you’re on foot you’re either going to be using a gadget or your grenades. If you’re fighting a heavily armored tank your best bet is to hit it in the rear as it will take nearly twice the damage as it would if you hit it from the front.
If you’re in a tank killing the enemy should be simple enough as long as you throw enough ammo at it. For the most part normal rounds (such as those from a machine gun) are ineffective against larger tanks and nearly pointless against anything else. So avoid trying to use a Humvee or an LMG to take down anything larger than a jeep.
Unfortunately for this achievement your squad kills don’t count for you. So any kills Haggard gets from killing a vehicle won’t count towards your total.
If you’re dying to get this achievement I recommend loading up the level “Air Force One” and playing through the level as normal. Not going out of my way to get any additional kills I counted 20 vehicle kills total. This achievement should be easily obtainable by the end of your first play through as long as you kill any vehicles in your way.