I don't know how common it is for a switch to not support VLAN or even to what extent it is about hardware. So far I've been out of luck with Zyxel NBG6617.
The page below says the switch is QCA8075 and VLAN is ¿.
That, in addition my efforts trying to get it working, leads me to think it's either not possible, or not yet if ever supported by OpenWrt. Does anyone know which one is it?