What Scott said, plus the line you are looking for in the /etc/rc file is "ifconfig vif0 <stuff>"
Note that if this line reads "ifconfig vif0 `hostname`-vif0 <stuff>" it is assigning the IP address found in /etc/hosts to the VIF. Change the address in /etc/hosts on the <HOSTNAME>-vif0 line to the new address before you reboot. As you are changing the subnet mask you will need to edit the netmask section of the ifconfig line in /etc/rc.
You will still need to manually run ifconfig vif0 <NEW IP ADDRESS> netmask <NEW SUBNET MASK> to make the change to the live system