It's not needed, why you want to set up an VLAN there?
I was able to assign a VLAN to the Guest network. I did it so that I could extend the Flint 2 Guest network over VLAN (802.1q) via the Flint 2’s LAN ports to my managed switches. Now, the devices connected to the managed switch ports that are assigned to the VLAN ID that I setup on the Flint 2’s Guest network are now on the Flint 2’s Guest network and get their IP’s from the Flint 2’s Guest network.
The way I did it was to first expose the br-guest bridge to the LUCI GUI by changing the /etc/config/network file to add the br-guest bridge as a standalone bridge device, and making the Guest Interface use the br-guest bridge instead of being the br-guest bridge itself. Then I did the rest of the configuration in the LUCI GUI since the br-guest bridge was now available to be configured in the GUI, and because I didn’t want to make any mistakes editing files anymore than I had to.