I have footage of a atomic mushroom cloud (pretty sure it's CG) that I got from Storyblocks. It's on a sky-blue gradient background, and doesn't have an alpha channel (sadly). However, the second half of the clip is a black and white alpha matte, where the black is the sky and the white is the mushroom cloud, and where the two colors meet it's nicely blended. I sliced the original clip into two, synced them up, and I can't for the life of me figure out how to make the black and white file knock out the proper transparency on the original, full color file so you can see another track underneath it. I've been searching YouTube videos and forums for two days now, I've tried Mask Generator and parent/child compositing, and nothing's working as it should. I'm hoping someone here can help me out, as I'm trying to get more into compositing and animation in Vegas and this is putting a damper on it. Thanks!