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(April 2010). A virtual IP address (VIP or VIPA) is an IP address that doesn't correspond to an actual physical network interface (port). Uses for VIPs include Network Address Translation (especially, One-to-many NAT), fault-tolerance, and mobility.
Clients should hit the VIP and be load balanced to a serverfarm, I don't see the interest of using NAT for the VIP address. If you want to use a private IP as VIP, you should do a static NAT of a public IP to a private IP on your FW or router in front of the ACE. Please clearify what you are trying to do and why you are trying to do that ;)