You said here in this duplicate post that your GPU acceleration is off, but we are still having difficulty understanding your issue in more detail. Please provide more info like @EricLNZ suggested above.
For HD and 4K specs, Vegas Pro 20 was the same as Vegas Pro 21's specs as far as I can recall (as was Vegas Pro 19).
In a nutshell, your computer is well below specs for Vegas Pro 20 which could well explain the issues that you are experiencing.
What is surprising is how you got Windows 11 to install on your up to 12 years old CPU - how did it pass the Windows 11 checks? - unless a hack was used to install it. I've got an i7 4th gen wihcih the official Windows 11 installation check refuses to install on because some of the W11 technology wasn't present in gen 4 CPUs
I doubt that there's a typo here (a missing '1' in front of '3550') because Intel's 'Ark" website doesn't show an i5-13550 version - and the i5-13500 version had 32 GB memory.
Any clarification on the above would be much appreciated.
I'm using windows 11 pro with Vegas pro 20 my ram is 8.00 gb. Cpu: i5-3550. Hdd
If you run Vegas on a machine that is significant below specs, nobody will be able to know if that is the reason for your black screens. But that will be the first assumption.