You need to stab the vehicle right before it explodes. If you see a helicopter pilot losing control and tumbling backwards into the ground or hill shoot it and you will get credited for the kill(s).
All my ground and air defence kills (except maybe about a dozen kills were from artillery for each category) are from actually using the weaponry. I have gotten over 30 kills IAR in FuShe Pass while defending at both mine entrances, especially the west mine entrance where I am under cover. That TOW alone gave me 24 kills! I was holding that flag by myself and blowing up Mobile AAs, full FAVs and LAVs, even tanks by shooting at the tread! It was so fun defending because there was an almost constant stream of enemies filing in. Anything that I didn't kill in the first salvo I would finish off with a SRAW or DAO if anyone bailed out. I was eventually overwhelmed by 8 infantry, 1 tank, 2 LAVs and an attack helicopter... Of which I killed the tank and gunner with the one-hit TOW to the tread, blew away two infantry with my DAO, pistol sniped a guy at the flag, blew one of the LAVs and got TV missiled by the attack heli. Where was everyone else? Who knows cuz I sure don't, but it was one of my greater Rambo moments.
TOW > all
The next round I joined the other team and I was invited to one of the squads I was killing last round and they were yelling at me (in a joking way) of how I kept killing them but glad that I was on their side now.
I have also found a lot of fun mowing down people with the mounted MGs. Especially the ones at the Airfield in Wake Island facing the Beach CP and the North base (although I wish there were two mounted MG nests instead of just the one for the north side.) Another fun mounted MG is found on Mashtuur City I think it's the MEC hotel CP where it is diagonally positioned and facing the Mosque. Kills Humvees and infantry crossing the bridge pretty quickly. They're also great to shoot down helicopters when they are within the shooting angle range.
Ground Defence is where it's at and has helped me gain many defend CP points as of late.