There wasn’t a system environment variable pointing to the Plugins folder at all.
This might have been the problem in the first place
This seems very fishy, as 3ds Max loads plugins from that directory by default. You shouldn’t need to specify it in an environment variable at all. If you didn’t have one to begin with (in either the System or User environment variables) then it’s quite unexpected that adding one should fix the problem.
I suspect something else was (and maybe still is) misconfigured on your system.
It seems Vray has its own environment variable already set-up.
No, V-Ray uses the recommended Autodesk Plugin Package system, which does not live in the main 3ds Max installation. Plugin packages are found in %ALLUSERSPROFILE%\Autodesk\ApplicationPlugins. That’s why 3ds Max was still finding it.