Some people have approached me about problems to activate vpn connections using Plasma NM. I have just found a workaround that enables vpnc (Cisco VPN) connections to work: just remove the /usr/libexec/nm-vpnc-auth-dialog file that comes with networkmanager-vpnc package. Yeah, that sounds strange but it works here. That seems an upstream problem, maybe that dialog is not working properly and yet blocking the dialog from Plasma NM. Do the same for the other vpn types, the workaround should work for them too.
Update: Ok, I think I have found the real problem and pushed a fix for it in Plasma NM's repo. I will release a new Plasma NM tar ball once the fix is confirmed. It is odd that the change was not required before and now it is, however the change makes sense.