For your purpose here you need to have each adapter in a different subnet. That's the only way to ensure that the traffic is separated. So, the "backup LAN" must use a different IP address range and subnet mask to your "public LAN". You will need to ensure that the static routes are correctly configured so that the default gateway is via the "public LAN".
VMS / TCPIP Services will handle multiple adapters in the same subnet, but there's no way to force specific outbound sessions over a specific adapter. VMS V7.3-2 and TCPIP V5.4 (and later) do all sorts of good failover things with multiple adapters (both outbound and inbound), but that's not what you're looking for. All you need is basic traffic separation - so it's back to a simple configuration with different IP addresses and different subnets for each adapter.
Traffic monitoring is probably best done at NIC level by looking at the NIC counters with SDA - or use an external LAN analyser. Later versions of TCPIP Services have facilities to look at the BG devices - I can't remember what was in UCX V4.2.
As has been mentioned - consider upgrading to current versions if you can.
Assuming that you also run DECnet then you might want to ensure that you don't have Phase IV addressing enabled on both adapters - if so then they will both run with the same MAC address. That's OK if they're totally separate LANs (or VLANs through the same switch). It's dfinitely not OK if they're both connected to the same LAN or VLAN. I assume that you won't need DECnet on the "backup LAN" - which will be fine.
Check for other protocols too and disable them on the relevant adapter too if appropriate (eg: clustering traffic, LAT etc.).
Cheers, Colin.
Entia non sunt multiplicanda praeter necessitatem (Occam's razor).
