If you don’t want the shots to overlap, you’ll have difficulty using a transition. On the other hand, you can drop the brightness effect on your video, then use the envelope (at the bottom of the efx window) to set the effect to begin where you want the effect to start. Then you move to the end of the shot and adjust the effect to how you’d like it to look at the end. Now when the video plays, Vegas will start the flash where you put the first box/mark and animate it smoothly till it reaches your endpoint setting. The envelopes are great ways to animate efx, moving alpha objects around and constantly adjusting audio levels.
If my explanation is confusing, let me know. I can run you through it step by step. Good luck.
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