I want to be able to overlay a video on top of another. I do not want to BLEND the two. I see how to do that. Instead, this second video should overlay my video and have transparent areas. Basically like the PiP objects. In fact, there is a specific one that might illustrate the point. Polaroid tree. It's an animated video that has holes for the video underneath to show through. What I'm using is much more complex than just a static hole though. It might be something like colored flowers dropping from the top of the frame to the bottom. I want the flowers to be 100% visible and my video behind it, where the flowers aren't, to also be 100% visible.

If I was in Vegas Pro, I'd drop in my overlay and set the Track to Mask mode with an extra setting or two and be done with it. I simply can't find any information that gives me a hint on how to achieve this so any tips are appreciated.

PS. In some cases my overlay may be a grayscale video, in other cases it may be colored with say a black background. I tried the Chroma Key and it sort of worked. I had a black and white clip. If I set the Chroma Key to white then my video showed through everywhere it was white. The only thing I couldn't do was change the black areas to another color. But this was a simple case. So again, I'm probably looking more for some direction than an exact answer. Thanks!