Your router is already a VLAN-capable switch—that's how it separates the WAN and LAN ports. It's a 7-port switch: by default it's configured as four LAN ports,

Your router is already a VLAN-capable switch—that's how it separates the WAN and LAN ports. It's a 7-port switch: by default it's configured as four LAN ports, one WAN port, and two ports are wired up to the SoC (https://openwrt.org/toh/linksys/wrt3200acm#switch_ports_for_vlans https://openwrt.org/toh/linksys/wrt3200acm#switch_ports_for_...). Using OpenWRT you can reconfigure the VLANs to suit your needs.