I've decided to do an updated version of my "how to render a video without the background" and "how to add that video into your Vegas Pro project" tutorial. It's still very close to the same steps, but my last tutorial was almost a decade ago and I kinda want to start sharing more tips and tricks for perhaps new editors.
First for those who want to know how to render a video of a scene where you've masked a character away from its background:
You need to choose a "ProRes 444" format when rendering your masked scene. Which one you need depends on the size and dimensions you want. I use 4K so my dimensions are 3840 x 2160, but if you want simple HD, then choose dimensions 1920 x 1080. The last few numbers are the FPS (frames per second) That number, like with the dimensions, need match your project properties, which you can find in project settings:
What is really important to know here is that you can NOT render your video into the usually preferred MP4 format as it does not support transparency. You need to render it as a MOV file, which means you need quicktime on your PC. It's a super easy and free download last time I checked.
Anyways, now that you have rendered your mask into a video, you can now put it into a new video project.
At first, this is what it's gonna look like: The background will simply be black, no matter what video, color or background you put in the video track beneath it. That's because you have to change that mask's properties first.
Right-click your mask and choose properties.
Then go to alpha channela dn choose "premultiplied." Remember to click OK after choosing it.
And voila, your mask's background is now transparent and you can put whatever background, color, pattern, etc. you want underneath it!
Do feel free to ask questions if anything I explained was confusing. Or if you have other editing questions! Remember, even though I personally work exclusively with Vegas Pro and Adobe After Effects, a lot of video editing programs function very similarly. So even though this tutorial is made within Vegas Pro, you should be able to do the same in, for example, premiere cut.
For those curious, here's the old version of my tutorial.