There are no decent "vapor-style smoke transition" plug-ins as far as I know of for FCP...but you can definitely build one in Motion if you have Final Cut Studio with one of the built-in smoke emitters, render it as an Animation codec file with a straight (Unmatted) Alpha Channel, import it into FCP and lay it in the V-track above your transition point.
I also would recommend the Wondertouch Particle Emitters for Motion (337 emitters for $149):
[
www.wondertouch.com]
Play with the parameters... have fun
- Joey
When life gives you dilemmas...make dilemmanade.