Weaver,
It may be a distraction, but it may also give you some ideas if I describe the way I've currently got my two modems (HG612s) set up.
Modem 1 - IP 10.x.x.201, LAN interface untagged, PPP packets tagged in VLAN 201
Modem 2 - IP 10.x.x.202, LAN interface untagged, PPP packets tagged in VLAN 202
(the HG612 allows you to VLAN-tag the PPP packets for VDSL (but NOT ADSL) - from a quick look at the user guide, I think the B10A allows similar, never tried the B10A though).
I then added static routes in both modems for my local LANs using 10.x.x.1 as the gateway. The alternative to that would be to add a NAT rule in the Firebrick (to NAT from 10.x.x.1).
The modems are then connected to a VLAN-capable switch, and then to the Firebrick.
This all simplifies things in the Firebrick - I just need a single "WAN" interface with the IP 10.x.x.1 and all my modems are accessible. I then use the "vlan" attribute in the "ppp" elements.
As for your configuration, if you're VLAN tagging the modem LANs, then yes, you'll need to add the appropriate "vlan=xxx" attributes to your "interface" elements (and then probably delete it from your "ppp" elements). Maybe that's all that's needed if a similar configuration was working before.