MP4 files don't support transparency. You need to render to a file type that does. Try mov with a codec that supports 32 bit for an alpha channel, or uncompressed avi. I'm not sure about other avi or other formats.
Is there a special reason for you requiring a transparent background?
My Vegas Pro 23 trial was interrupted by work that I was doing on the last day of Windows 10 support, so I initiated a new trial today on the beta version. All went well except that I don't have any render options on Vegas Pro 23-302, either.
Background
I've created output from Sonic Foundry, Sony, and Magix products since the mid 1990s. I've done system-level problem determination on the largest systems on Panet Earth in my past life, this morning I solved a software problem on another application and this afternoon, I replaced the belt on my daughter's vacuum cleaner.
After all that, I can't use Vegas Pro 23-302 to output a demo file for another would-be Vegas Pro user. I'm going to go watch mindless TV.
@RogerS Thank you, Roger. For a trial, I usually click first and ask questions later. This morning, I was offered 720p AVC if I uncheck all the filters.
Probably not going to allow me to learn much about the new Render panel if I'm offered only one option in the trial.