I've had a vlan tagged connection for a while now, and as Nick reports the config file will be in sysconfig.
I've never had a great deal of success with the web gui, and always did the config from the command line.
After changing anything, I always did a network restart, followed by a firewall restart.
Sorry for the delayed response, was at the Dayton Hamfest - my other "hobby" :-)
Cheers
Paul