You can store them where ever you want. To retrieve them you can drag and drop them to the diagram or use "Select a VI..." from the function palette. The later will open a file dialog that will allow you to navigate to where you stored the vi. Another way to do it is to store your vi in the LabVIEW\user.lib directory, then you can retrieve your vi from "User Libraries" in the function palette.
Hope this helps
Brian