Just some tips for "Destroy a vehicle with xbow". You'll obviously need HE Bolt.
The range is quite short, and the arrows travel quite slowly, so any type of airborne vehicle is basically out of the question.
Next, I've tried to use the HE Bolt against a tank, just to see what would happen. After 3 arrows to the back of a full tank, I got a vehicle destroy assist of 41, meaning each bow does about 12-14 damage. Considering that a vehicle becomes disabled around 20, unless you catch someone really stupid in the driver's seat, they'll jump out before you destroy it. For the most part, it's a lost cause, but occasionally worth trying on a smaller tank.
What you are left with are Humvees, Buggys and Quads (and technically the Skid Loader on Wake Island). Humvees are tough so I don't recommend them. Buggys will be disabled (but not destroyed) in 1 shot, meaning the driver will have to stay inside and then explode. Quad is 1 shot.
Choose a map like Gulf of Oman, Strike at Karkand, Wake Islandor even Bandhar Desert, and just wait for someone on a quad within short range, and take them out! You can also grenade a buggy, and then camp somewhere around it, hoping that an enemy will enter it, and you can shoot it as soon as they enter exploding it immediately (this is how I got).