Your opnsense will have WAN (ethernet port) and your LAN side will be all virtualized. There's no problem having VLAN 10 with 192.168.10.0/24 for your main vms and then VLAN 20 with 192.168.20.0/24 for your VPN machine. Setup deny rules in the firewall to stop the VLANs from communicating.
If this is inside your current home network you will end up with double NAT though.