@naiw the instructions from Vultr sound like targeted at Linux systems (ip a, /etc/sysconfig/ -> RedHat). You may ask them for FreeBSD instructions or better for pfSense.. But I don't think automatic config will work with pfSense at all.
They assign the second IP to your instance and you have to manually create an alias, as @stephenw10 mentioned.
More infos (although old, 2016): On this forum: Two totally separate IP's on WAN - how to configure on VULTR. The result was the same, create an alias.