Do you have any links with examples of what you're after?
Without really knowing what you're trying to achieve, and only guessing based on your description, I would guess you could use a gradient mask, full white (or whatever color) to transparent, set the blend mode to add, or overlay maybe.. then toggle it's opacity with keyframes.
Whether you can do that in vegas or not, I don't know.. though you could really just create a transparent png with photoshop, or gimp, or whatever, and use that I'd think.