Here's a link to a demo using HappyOtterScript's different alpha options. https://tools4vegas.com/renderwithalpha-demo/ The demo's a bit dated since one no longer needs to configure the FrameServer--it's now done internally. You can see the relative file sizes with various avi and mov options. Potentially, the Motion-Png codec within an mov container might be of interest since it produced file sizes roughly half AppleAnimation. In comparison, DnxHd inside of Vegas produced huge files.
Just curious. Why not avi's since there are quite a few options? MagicYUV, UtVideo, GrassValley HQX? Certainly, a lot more compatible with Vegas.