Update: It appears that this challenge only works when the aircraft doesn't die. I tried again for a couple hours on this one using the SLM launcher and after shooting a couple of scout helicopters (which took more hits than other aircraft) and it not going down on the first rocket, the damage counted. The first one counted for something over 1800 damage, so it only took 2 damaging hits for the challenge to be over
I would advise if next time there is clarification on the challenge to title it differently. You have to do damage to kill vehicles, but if it doesn't count and people are just killing aircraft and never getting the challenge done, they won't do the challenge and just hate the progression more
