I have solved this, but had to dig to figure it out, so figured I would post here. The issue was when I tried to install this older version of SVMSP 11 on a new Windows 10 computer. I had my old installer program, but when running it, it would say that I needed .NET Framework 3.5, and when I would OK that to be installed, it would get almost done, then give me an error message "Download Corrupted", and would go no further. One suggestion I found on this site was to download and run a separate installer for Framework 4.0, but when I did that, it would just tell me I didn't need it because my computer was up to date.

I found someone having an issue with .NET FRamework 3.5 (though not specifically for Sony Vegas), so I followed the advice there:

In Windows 10 search, type: Windows Features

You get a menu that lets you turn Windows features on or off.

Turn off .NET Framework 4.6 Advanced Services

Turn ON .NET Framework 3.5 Services

Windows then installs what you need, and after that, run the Sony Vegas installer, and all is good.

I have not tried it yet, but I might try to turn on the .NET Framework 4.6 again, now that I am installed, and see if all works fine, just in case newer programs need that version.