And VLAN-tagged only upstream of the modem ?
Yes, although the HG612 has an option to vlan-tag the packets downstream. But normally you would configure it to strip them.
I had a play with that feature recently (when I had one VDSL line, and one ADSL line, both using HG612s). The option worked fine with the VDSL, but didn't work with ADSL.
But it did allow "daisy-chaining" the HG612s in following configuration:
1) LAN1 on HG612#1 connected to my router
2) LAN2 on HG612#1 connected to LAN1 on HG612#2
3) VDSL vlan-tagged
4) ADSL not vlan-tagged
5) Both HG612s configured with a LAN IP in the same subnet
6) Both HG612s configured with "port binding" disabled for LAN1 and LAN2
The port on my router (Firebrick FB2700) was configured with an IP in the same range as the HG612s, and both PPP connections on also on the same port (the VDSL with a VLAN tag).
It all worked fine. It's a shame the ethernet ports on the HG612 are only 100Mbits/s, otherwise I would probably use the same configuration with my current setup of 2 VDSL lines (both syncing at approx 80/20).