[sm=confused.gif]Vapor trails depend on the relative humidity in the air + the sudden depression suffered by the air stream over the minimum press zone over the wing or inside the vortex generated in a wing tip.

[sm=drowning.gif]To make vapor trails, we must increase the airspeed &/or the "G's" to increase the lift force and consecuently the depression in the air that make posible to reach the dew point of the air.

Maybe we can get the required air depression using a specific shape or device that accelerate the air or generates a vortex similar to the one that occurs in the big jets in the wing tips or in the flaps tips.

[&:] Also we can try to lower the temperature in the airflow over the wing, maybe with the sudden discharge of a bottle containing gas under pressure guided through a tube over the wing.

[:@] Finally we can get the vapor trails for sure increasing the airspeed to higher levels, of course in humid conditions.

I think that now there are just a few model jets (military UAV) able to fly at such airspeeds to produce vapor trails. Sometimes the vapor trials are the result of a deep imagination of some spectators or pilots.

[:-]I don't think a fuel leakage in the fuel vent can produce such a vapor trail, if we think that the fuel leak is very small and unusual, almost imposible in a normal flight condition.