They are not meaningless, they are clearly labelled.
The physical interface depend on the router (eth1 and eth2 are usually wireless and are usually tagged WL and eth0 is usually the switch).
Vlan1 is your LAN, including wifi
Vlan2 is your WAN, everything going to/from internet, tagged WAN
br0 is also your LAN, should report the same as Vlan1
If I understand you correctly, you want to monitor per client, which is not what the bandwidth monitor is meant to do.
What you want is IP Traffic monitor, it is present in Toastman's and Teaman's builds.
You can see screenshots here: code.google.com/p/tomato-sdhc-vlan/wiki/TeamanIPTraffic
