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
After the iso is downloaded and we get back a prompt, we can then:
chain http://your_host/memdisk iso
You can even make an iPXE script:
#!ipxe
initrd http://mfsbsd.vx.sk/files/iso/10/amd64/mfsbsd-se-10.1-RELEASE-amd64.iso
chain http://your_host/memdisk iso
I used these steps on iPXE Contabo boot and worked, thanks!
Self hosted
memdisk
from thesyslinux
package of course.