Could probably be done with a script... someone here with more knowledge of how to code that up might be kind enough to post a script here that can do that... the hard way would be to cut the video out at those intervals. or get a video that flashes black and white from youtube using screen record and speed it up/slow it down by control + clicking and dragging the edges of the clip to shorten or lengthen it in VEGAS... then you can set the composite mode of that track to multiply, and that will black out the video while letting it show through while white. The best way would be with a script that cuts the video out at those intervals, though. Many ways to do things in VEGAS.