I had to downgrade my Vegas version due to stability issues with 21 build 300, but projects generated by 300 won't open in the older versions. After a period of research, I just found a solution. Here it is.
- Back up your project files in advance to avoid any possible problems.
- Download and install HxD (https://mh-nexus.de/en/hxd/), open your project file with it.
- Change the value of "00000045" from "50" to "00" and save.
- Now the new one can be successfully opened with older Vegas versions.
I hope my post can help someone who is "suffering" from 21 build 300.