type "transition" into VEGAS Content search, add a new video track to put the overlay on that is above the tracks you want to do the transition on, then use the "screen" compositing mode in the track settings for the overlay track. There's a bunch of really cool transitions in there you can choose from, some that perfectly match what you are after, but a ton of others as well that are also really cool. If the transition is too slow, remember you can slow down or speed up events by holding ctrl and clicking and dragging on the edge of the event to make it shorter or longer. When time-stretching video to be longer, consider using the "optical flow" resample mode in project settings to make the slowmo smoother.