First, make sure that you have Advanced View enabled within the Site Overview of the SimpleWAN device in question.
Next, choose vLANs from the navigation menu as seen below.
After that, click the “+” button at the far right of the menu that comes up after clicking the vLAN menu option mentioned above.
Next, you'll populate the fields in the form that pops up with the correct information for your vLAN configuration. You'll be filling out the name field, the parent interface where the vLAN will live (LAN/WAN), the vLAN tag, the IP address for the vLAN interface, the subnet mask of the vLAN interface, and finally whether or not the vLAN interface should be enabled or disabled at this time.
The last step will be creating a DHCP server for your new vLAN interface. Please select DHCP Server from the Navigation pane in the Site Overview page. Click the “+” at the top right of the section entitled “DHCP Server” on the page that is brought up from the Navigation pane. Choose your newly created vLAN interface from the drop down menu that is presented. Finally, choose your start and end address from the available range you chose from your Subnet selection when creating the vLAN. Click save, wait for the SimpleWAN device to sync, and now you can start using your new vLAN! (assuming you setup your switch correctly!)