Yea, just open up the effect window. Then on the time line under the effects insert keyframes where where you want the effects to start and stop (making sure to insert a key frame for the start and finish of the effect) and then another keyframe after it, to bring it back to normal. However close the keyframes are to each other, from the finish of the effect to back to normal, will effect how long it will take to get back to normal.

I should have said I am using Vegas Video 4.0

I went to the Help section and looked in the Glossary and found nothing on Keyframes. I also did a search run in the help sections on "Keyframes" and got no results.

This is what I am looking at....
http://tinypic.com/u3n7q

Let the clip play. And then start fiddling with the FX controls. You will see there is a tiny timeline, underneath the FX control window. As you adjust the controls, you will see diamonds start to appear don't there. These are keyframes. Keyframes allow you to control an effect over time.

For example, you put a light ray effects on a clip and in the beginning of the clip, the light ray are weak and the light is coming in from the right. you can then put a key frame at the end of the clip, that is strong and coming from the right. As the clip plays the direction of the light rays will change they they will grow more intense.

Yoiu might want to try thr Vegas Movie Studio forum

http://mediasoftware.sonypictures.com/forums/ShowTopics.asp?ForumID=12

Vegas Movie Studio doesn't have keyframes. The easiest way to accomplish what you want is to Split the clip where you want the effect to start and Split it again where you want it to end. You can then apply the effect to just that section.

You would think that someone would have come up with such a simple answer sooner. Don't I feel sheepish.