I see, what you need in that case is to enable the the "X-Forwarded-For" header in the http profile of your VIP.
You can also do the same thing using iRule below and then associate the iRule with the VIP, this iRule will basically insert a new header called X-Forwarder-For into your users HTTP Request Header:
when HTTP_REQUEST {
HTTP::header insert "X-Forwarded-For" [IP::client_addr]
}
You’ll then be faced with the small problem of how to extract the client IP from this new header called X-Forwarder-for into your backend server logs, there are two ISAPI filter solutions if you are using IIS, one solution originates from MS MSDN and the other from F5’s Joe.
If you are using Apache then its easier to extract than IIS. Do a search for IIS X-Forwarded-for in Devcentral for these extraction solutions.