By default, the plug-ins, libraries, and workspaces all get installed in your Vectorworks user data folder.
On the Mac this is: ~/Library/Application Support/Vectorworks/{version number}
On Windows, this is: /users/{user_name}/Application Data/Nemetschek/Vectorworks/{version number}
You can confirm the location that Vectorworks looks for user data. Open Vectorworks preferences (Tools>Options>Vectorworks Preferences…). Switch to the User Folders tab. You will see the setting for your user data folder.
Note, if you are confirming installation via the Plug-in Manager, compiled plug-ins appear in the Built-in tab, not the Third-party tab