1:1 NAT and VIP's

[geom]

Ok, so I'm confused when you need to use a VIP.  I have read several 'how to's' describing setting up proxy-arp's/carps's and using a VIP.  I've even read a comparison about monowall's 'Server NAT' being the same as 1:1 + VIP.

In my experience though I did not have to use a VIP for additional some static IP's on my WAN interface.  I simply configured a 1:1 NAT, some firewall rules to let in certain ports from the Internet, and everything seems to work.  Is that right, or do I need to make a VIP for some reason as well?

I did make a VIP for some clients outbound and used it in the advanced NAT configuration.  That makes perfect sense to me.  Just confused about 1:1 NAT's and if they need a VIP for some reason as well?

Thanks.