If I remember correctly the F-16 that the Virtual Viper East Coast Demo Team flew at last years VFAT had vapor effects, I have no idea how they were added though.
I guess you could link the effects to the animation of for instance the elevators, giving you vapor effects when the elevators are at a certain position, however this would give you vapor effects on the ground as well, so this isnt exactly ideal... Im not sure if there is a way to get the effects to react to g-forces, angle of attack and so on right now, so lets just hope that ED adds some vapor effects themselves soon.