Monitoring between Router/Firewall to the Subnet

I have a quick question. I'm setting up a Monitoring system on my network. I have a router (virtual) in place and a firewall in front of the server. In between the router and firewall there is a vlan. So off my interface for the router I have subnet address for the interface i.e: eth0 (goes to virtual router) eth0.115 (goes to subnet).

Is it necessary for me to monitor the subnet in order to get a proper read on any bandwidth or packet loss, especially since it's in between two nodes I'm monitoring already?

Thank you! Much appreciated!