Vulching: Shooting guys that are on the runway, or just lifting off. The term likely came from the circling of the runway as if you were a bird (vulcher) waiting for prey.
Honestly, good vulching requires quite a bit of skill as timing and angles are very important as well as the ability to monitor multiple spawn points on the runway at the same time. Shooting someone up while on the runway is not very tough but getting your pattern such that you can respond when the guy first spawns and shoot him full of holes before he has a chance to react, all while not trying to run into the ground, is tough for some people.