It’s not exactly the same, but did you try the Sony Vegas Pro Transitions:Flash:Hard Flash? It very close to what you are looking for and is very EASY to use. Just overlap your two video events for whatever time you want this flash transition to occur, then drag the Hard Flash transition onto the overlapping events area. Done.
More complicated would be to put a Sony Media Generators:Solid Color (white) event for the amount of time you want the flash to occur. Put this on a new track above the one you want to have the transition for. Set the tracks Compositing Mode (on the track header) to Dodge. Next, right-click the header and select Insert/Remove Envelope: Composite Level. This will put a Composite Level line across the top of the track. On the left edge of the white solid event, right-click on the Composite Level line to Add a Point. Add a point at the mid-point of the white solid event, and one final point at the right edge of the white event. Drag the left and right points down to Composite Level = 0%. Doing all this will automate the white “flash” to go from 0% visible at the start, to a maximum of 100% at your mid-way point, then back down to 0% at the end of the event. Just depends on how much control you want to have of the process, and how much time you want to spend.
Andrew Lenczycki