If a VPN plugin is not installed the respective VPN option is available anyway. One can create the connection, and when clicking "connect" it will just do nothing at all, not even show an error message. Either plasma-nm should show an error message, or grey-out the unavailable options. This is true at least for openVPN and pptp vpn.
*** Bug 394680 has been marked as a duplicate of this bug. ***
There should be now an error message when there is a missing plugin.