I'm facing a similar issue. We have 3Com Switches and have defined various VLANS on the switch. NPM finds these VLANS as resources but does not collect any traffic statistics from the VLANS. I'm able to collect the stats for the various ports on the switch but not for the VLAN Interface. I'm also running Toolset V8.
Is it possible to collect traffic stats for defined VLAN interfaces on a switch?
Regarding monitoring VLAN stats... It all depends on the switch. If the agent on the switch presents stats in the IF or IFx MIBs for the VLAN, NPM will collect them. If the Agent doesn't provide them, there not much NPM can do.