The problem is that Vegas only supplies single images to plugins instead of entire clips. This is true for all plugins. The work-around is to:
1. Render from Vegas the small clip that you want to pass to FX.
2. Add the FX plugin to the clip (if you have not already done so).
3. Enter Boris FX and look for the video that came from Vegas (it will be a reference to media “V1”).
4. Replace the reference to “V1” with the name of the clip that you rendered in step 1.
5. Proceed as usual. The results from FX will be automatically returned to Vegas. That is, no additional rendering “tricks” are needed.