Couple different ways to do that:
If transitioning between two clips, use the FLASH transition. There are a couple different types of flash transitions - Soft Flash, Hard Flash, and a couple other (haven't used Vegas in a couple months, sorry).
If you're adding flashes to the middle of a clip, like done several times in the Propaganda trailer, you need to know how to work with keyframes. Add a brightness/contrast effect to the clip, and use keyframing to heighten the brightness throughout the event.
A simpler way to do this is just to use a crossfade, and change the color settings so that it flashes white instead of fading to black.