IP traffic monitoring and VPN client

I'm facing an issue with IP traffic monitoring when I'm using my VPN client (Tomato Firmware 1.28.0000 -3.5-140 K26ARM USB AIO-64K with Advanced Tomato on r7000) : I can see the upload traffic (TX) but not the download (TX) one. When I stop the VPN client, I can monitor both uplaod and download traffic...

Is there any solution to monitor download IP traffic comming through the VPN client ?

Thanks for your help