impose Session limits by ipadress on VIP

Have a look in the Connection Rate Limit Mode in the virtual server configuration. There are multiple modes there, just check the help menu.

I guess the one you are looking for is this one.

"Per Virtual Server and Source Address: Applies Connection Rate Limit Source Mask to the source IP address of incoming connections to this virtual server, and applies the rate limit to connections sharing the same subnet. The Connection Rate Limit Source Mask specifies the number of bits in the IP address to use as a limit key."