Not sure if this is the "vapor cloud" in question in this thread but the "vapor cloud" you're talking about isn't necessarily caused by the "G-forces" (aka load factor). When the aircraft pulls into the vertical, the lift created by the aircraft is dramatically increased in a short period of time. This causes the density of the air on top of the lifting surfaces to drop accordingly (see Bernouli's Principle).
It's a similar mechanism to the transonic aircraft "squeezing" the moisture out of the air in straight and level flight (the "vapor cloud" discussed in this thread), just accomplished differently. The G-forces are just a side product of the aircraft creating that much lift and causing a dramatic change in direction in a short period of time (see Newton's First Law of Motion).
I may just be talking out of my ass though.
