If your custom iso is not mounting, and your console is at an iPXE shell, you can:
initrd http://mfsbsd.vx.sk/files/iso/10/amd64/mfsbsd-se-10.1-RELEASE-amd64.iso
While the iso is downloading, vultr's iPXE does not include memdisk, so we have to upload it ourselves to another host.
Here's one I found public, though I wouldn't recommend using it: http://preseed.panticz.de/ipxe/memdisk