I don't know if this will answer your question but... here it goes. In Spain, Movistar uses VLAN 2 for IPTV. The configuration is (in my case, RB750GL, and a netgear switch hooked to eth4, and the decoder to that switch):
Eth4 is a slave port to Eth2.
VLAN2 has been created under Eth1 (my WAN access).
VLAN2 is given the address that Movistar provides for IPTV (in IP->Address)
The decoder is given a specific ip with certain options (I don't think they apply to your case)
VLAN mode disabled and VLAN Header "leave as is" in every port.
In Firewall Rules, NAT:
chain=srcnat action=masquerade out-interface=vlan2
And a special rule for Video on Demand (which is why the decoder is given a static ip):
chain=dstnat action=dst-nat to-addresses=[Decoder_IP] dst-address-type=local in-interface=vlan2 log=no log-prefix=""
Finally, in Mangle:
chain=postrouting action=set-priority new-priority=4 out-interface=vlan2
Again, all this is valid for Movistar in Spain, which uses a ppoe-out connection with a ONT.