Classic Vegas being difficult! A similar issue occurs for me when making a nested timeline. As soon as I tab out and back in, Vegas instantly crashes.
If changing the Dynamic RAM Preview to 0 doesn’t work, you could try to set vegas to High Perfomance mode.. If Windows is set to optimize performance based on active applications, background tasks may be deprioritized and cause issues while preparing. Go to Control Panel > Power Options and set it to High Performance. Also check Vegas.exe in Task Manager and set its priority to High.
If this doesn’t work, I would recommend to revert these changes. Not sure if they have any negative impact on Vegas that I haven’t discovered yet.
Let me know if this works for you!