I've been successful in adding VIP's using these commands for each VIP in the pfsense config (xl0 is my WAN interface):
<system>...
<shellcmd>ifconfig xl0 10.1.1.254 alias</shellcmd>
<shellcmd>route add 10.1.1.0/24 -iface xl0</shellcmd>
...</system>
Note I then had to add manual outbound NAT for each VIP created (192.168.10.0 is my LAN subnet):
WAN | 192.168.10.0 | * | 10.1.1.0/24 | * | 10.1.1.254 | * | NO
…
WAN | 192.168.10.0 | * | * | * | 192.168.0.2 | * | NO
(The second entry is the actual WAN interface IP)
I set this up a while ago, and foolishly didn't document any of it! So I hope this makes sense to you.