Just posting as this might be a useful tip for others, and it might lead to more info on how Vegas uses RAM. My preferred workflow is to have two project windows open. One has all my rushes cut up into sound bites and cutaways, all with notes to identify them in the timeline. The other project is for the finished edit.
To build the edit, I grab clips from the rushes project and Alt-Tab then copy them in to the edit window in the order I want them. It's a really quick way of keeping track of where you've reached in the rushes and the edit. I know the new project clips folder in the Hub would do this as well, but that only gives you complete clips, not the labelled snippets I need.
However, as I've started to use more and more 4k 10bit HEVC footage, I've noted my PC struggling and often crashing when switching between complex projects. Today I noticed that if I keep playing the timeline in one window while I copy, alt-tab and paste to the other project, there is no lag and no attempt for vegas to reload either project and no crashing. It's made my editing way less problematic.
I guess it's because keeping the timeline playing stops Vegas from trying to swap the project files over in RAM, but crucially it still allows you to paste and save into the non playing project.
Maybe there could be a global 'memory focus' setting that allows you to have multiple projects open and only keep one of them in active RAM? That maybe not how it works at all, so it's just a starting point if anyone else really knows how this works.
V20 pro on a PC with 64gb RAM. 5900x+6700XT