I don’t think you can do this, without potentially having issues. Some people have done this:
Open the registry Start > Run > Regedit.
Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion
Open “ProgramFilesDir” from the list in the right pane, and change what is normally C:\Program Files to whatever it is you want (you probably have D:\Steinberg\Cubase 6\VSTPlugins\Guitar Sims…). You can then do the same for "ProgramFilesDir (x86).
However, there is a potential problem in doing this. Read:
When you install a Plugin the installer checks the vst plugin directory in you registry. This entry is set by what you enter for the first VST Plugin you install. After this this path will always be pre entered when you install another VST Plugin. To change this registry entry you must open regedit in windows.
Push the keys windows + r to open the run dialog and type regedit.
The Registry Editor will open
Backup the registry (file, Export…, export range = all)
Expand HKEY_LOCAL_MACHINE
Expand SOFTWARE
Find and click on VST
You will see two entries one says default – (value not set) and the other might for example say VSTPluginsPath – C:\Program Files\Steinberg\Cubase 6\VstPlugins double click on this one.
Modify the data to the desired folder address (" C:\Program Files\Steinberg\VSTPlugins".)