so whats the trick to make real nice vapor trails?
Similar to tire marks.
As the trail generator moves every couple frames you mark current location as a new point in a line. A geometry making class turns those line segments into quads or tri-strips based on several parameters such as the ground surface for tire marks or for vapor trails some quick hack and up and down quad and a X-Z plane quad crossing at the actual line location.
The best vapor trails are a continuous strip that is updated every frame to be rendered flat to the view, similar to particle systems. In fact, a modified particle system is a good place to start. Sometimes a series of close particles can do the trick nicely as well.
There is also the possibility of using a particle engine for vapor trails. If you mean a vapor trail is something similar to a smoke trail left by a rocket, plane or something.
Use a trick like explosion clouds to create a black and white “cloud”, make the image translucent, and plug the result into a particle engine. By making the cloud brighter or darker, you can control whether it looks like smoke or exhaust. For a crashing fireball effect, run an animated explosion effect at the particle source. Good luck! 