Sorry for the confusion. I had forgotten that virtual interfaces won't show up in get system so the interface number can only be seen for those in get interface.
And you are correct the traffic counters is available on the srx but not on screenos.
Unfortunaely the nspflag refers to network service processor which is where the flows are processed as flows so this is not tied to specific interfaces but is an overall resource for all flows to share and is related to the maximum session limits per platform.
So the vlan or the source/destination ip addresses are the only clues to narrow down the sub interface information.
the other clue is routing tables. Sessions are based on the ingress interface and the egress interface which are determined by the routing table. So if you look up the ip addresses in the flow in the routing table this will return the interface information you are looking for too.