OPNSense vnstat

System > Firmware > Plugins, search for vnstat, and click the + icon next to os-vnstat.

Go to Services > vnStat in the main menu.
Enable the service checkbox.
Select the network interfaces you want to monitor (such as WAN or LAN).

OPNSense Bridge

  1. Interfaces: Assignments » add igb[n]
  2. Interfaces: Other Types: Bridge » create Bridge
  3. Interfaces: [OPT(n)] » enable Device
  4. Interfaces: Other Types: Bridge » add member Device(s)
  5. Interfaces: Assignments » change LAN device to Bridge
  6. (add old LAN Interface to Bridge)