You cannot put 10.0.0.0/8 on an interface and use 10.100.5.1/24 to give to OpenVPN clients. Those subnets overlap.
If you, for example, assign the IP address 10.23.56.34/8 to a host on em2 and it has traffic for 10.100.5.1 it is going to think it's on the same subnet and not send the traffic back to the firewall to be forwarded to the OpenVPN client.
To tag traffic on a pfSense interface, you must first create a VLAN on the interface Interfaces > (assign), VLANs tab, then assign the interface to VLAN XXX on em2 in Interfaces > (assign). Then connect em2 to a switch port or device that expects traffic tagged on VLAN XXX.