Love it! After some fiddling of Virtualbox settings (network cards etc) and turning off the vigor 100's dhcp server, it all just clicked last night and I now have a functioning pfSense install in Virtualbox under an Arch Linux host (64-bit all round).
Hardware is an i5-750 and I've given the VM 1 core and 768MB of RAM on an 8GB virtual drive. NIC is an Intel i350-T4, with eth0 (em0) running bridged as WAN and pppoe into my ADSL2 modem.
LAN is eth1 (em1), also bridged, into an asus-rt68 now just acting as wireless AP and wired switch. To get the host to connect I've "looped back" from the switch into the NIC's 3rd port (eth2).
Static IP addresses on the LAN, static routes, ddns (afraid.org), ntp and port forwarding are all straight forward - whoever said pfSense had an overly complicated interface? Actually easier than the Asus to set a specific DNS to a specific MAC - no scripting required.
Tonight's task is to get my openVPN server up and running - I want to import my existing rsa keys, which looks slightly convoluted
And perhaps I'll try to get this running in qemu/kvm, if there is any advantage to doing so. I had almost planned to pick up an atom mini itx box, or recycled old PC (i2500), but as this is working so well, and this host runs 24/7 for other purposes, I'm glad the Virtualbox option seems so good. This is especially as I'm only running this as a home network (1 desktop, 2 laptops, 2 smart phones and a Roku) - so not exactly taxing. with 100/20 fibre on the way later this year though, I didn't want to be maxing out a consumer router (the rt-ac68) with AES-256 openVPN running.