Try to edit the Connection Name of a connection, and nm-vpn-properties crashes upon hitting apply.
Please specify exactly which versions of NetworkManager and NetworkManager-vpnc are installed. Also be sure to test after a full reboot because the older version of nm-system-settings could be screwing up the tests. (I asked this Friday night, no details yet, and I can't reproduce this.)
This is with NetworkManager-0.7.0-0.9.3.svn3623.fc9.i386
I'm using NetworkManager-0.7.0-0.9.3.svn3623.fc9.x86_64 here. It doesn't crash here, but it does corrupt what is displayed in the VPN Connections menu. Closing that menu and opening it again it is displayed properly.
found and fixed in upstream applet svn r713