So, you want to make eth2 have a PVID of 845, and eth2 is not on the switch? That's it?
I think PVIDs and VLAN tags only apply to switch interfaces - if you're routing then there isn't a need for VLAN tags. I haven't used this feature much myself, but counterintuitive as it may seem I think what you want to do is add the port back to the switch, set the switch as vlan-aware, and set the port PVID to 845. You can set vif 845 on the switch with its own routed IP.
Otherwise, you can keep the port off the switch with its own address. Add firewall rules to allow the port to talk to devices on vif 845 and not others, as you like.