nm-applet and gnome-shell appear to be doing the right thing: gnome-shell adds its own LIBEXECDIR if the path isn't absolute, and nm-applet always strips out the path to use its own LIBEXECDIR (which would be the right one anyway).
I'll look at all the VPN plugins and test if adding the libexecdir as path to the auth-dialog would be sufficient to correctly fix this.
nm-applet and gnome-shell appear to be doing the right thing: gnome-shell adds its own LIBEXECDIR if the path isn't absolute, and nm-applet always strips out the path to use its own LIBEXECDIR (which would be the right one anyway).
I'll look at all the VPN plugins and test if adding the libexecdir as path to the auth-dialog would be sufficient to correctly fix this.