It does make sense to do the vlan tagging on the (OpenWrt-) router, instead of making the modem do it for the router - and for several configurations it's even necessary (e.g. 'internet' on vlan tag 7, IP-TV on vlan tag 8, keeping the modem management interface untagged, etc.). How that is about to be configured on the modem is another question, I'm personally doing exactly that (vlan tagging on the router, not the modem), but using a different VDSL modem - so I can't really help you with the vmg1312-b30a (your wan6 configuration should use option ifname '@wan' though).