If the traffic is hitting the VIP but failing, I think the clients have a route through LTM for the SNMP traffic.
If only 5% of the SNMP traffic is being parsed successfully through LTM, I'd think capturing a failure in a tcpdump should be relatively easy.
Are you saying that the trace on the client VLAN and the server VLAN for a failure shows the same data being received and sent by LTM?
What are the symptoms of a failure? Does the SNMP server receive the packet(s), but fail in parsing them? Or does the server not get the packets at all?
Thanks,
Aaron