I receive the error message that Vegas 19.0 is already installed and to uninstall it before upgrading. This is using the automated upgrade method. I've tried a few times.
I fought this issue for a week or so. I finally uninstalled Vegas and reinstalled from the My Products page (https://www.vegascreativesoftware.com/us/support/service-center/). It went through the complete install and the message went away. I just upgraded to Build 424 yesterday with no issues. It was a long process, but it did get fixed.