See what wblock@ has on his homepage: http://www.wonkity.com/~wblock/docs/html/backup.html
These sections have what you are looking for:
dump Via SSH
restore(8) Over A Live System
Thanks for the suggestions. What I'm not really sure about is what to backup. I guess I don't really need to backup the base operating system because that can always be re-installed. What I'm confused about is where are 'my files' stored? ie the data and any programs/configuration files which I have installed after the base FreeBSD install.
The base OS is so tiny (a couple of GB at most) and changes so rarely, why wouldn't you just backup everything? Recovery is much simpler that way. Just boot off a LiveCD, format the drives, restore the backups, and reboot. No muss, no fuss, no worrying about missing a file somewhere, no worrying about getting things setup and installed the same way.
You don't need physical access to it for the backups. You can just run rsync-over-ssh and copy the data off to wherever (another VM, a USB disk attached to your home computer, etc). Or, if using ZFS in the VM, just use "zfs send" for each filesystem.
For recovery, you use the vSphere client to connect an ISO image file of a LiveCD (like mfsBSD) and use that to boot (the vSphere client gives you "physical access" to the console as if you were sitting in front of physical system). Partition the virtual disks, create the filesystems, mount the filesystems, and then use rsync to copy the data back from the backups to the disks. Update the boot blocks, and reboot into the VM. (Glossing over a few details, but that's the gist of it.)