I do not mean when/if I reinstall SONAR but when I install 3rd party plugs.
 
Is there a registry entry that I am unaware of?
 
Or is it the InstallDirectory key in
 
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\VSTPlugins  

Some manufacturers use the value in HKEY_LOCAL_MACHINE\SOFTWARE\VST\VSTPluginsPath as the default install location. Some maintain their own value in the registry.

Thanks Steve. I've at least reset that to my common VST2 64bit folder.

Seems like every vendor has their own scheme for persisting the install location. Even worse, a few don't even bother and simply default to \program files\vstplugins, steinberg\vstplugins or a top-level folder called vstplugins (lookin' at you, u-he; it's not 1995 anymore).
 
After ending up with duplicate DLLs on several occasions, I just gave up and now let those lazy vendors stick 'em wherever they want. Even though c: is my smallest drive, VSTs just don't take up enough space to warrant fretting over.
 
Far more impactful is where sample libraries go, so that's all I worry about. I make sure there's at least enough room on c: for the initial install and then move them (pet peeve: why do vendors even bother with installers for Kontakt libraries anyway? Gimme a zip file, I'll figure it out!).