Reroute traffic based on request content

Most certainly. But you may have to consider other factors. Is this a single atomic request, or will the user make multiple requests? Will all of the user requests contain the information you're looking for?

Let's assume for a moment that these are atomic requests, and by "VIP" you actually mean servers, or a pool of servers. You could simply use a string function to parse out the content you're looking for and make a load balancing decision based on that. Something like this:

when HTTP_REQUEST {
    if { [HTTP::header exists Content-Length] } {
         Collect the payload and trigger the HTTP_REQUEST_DATA event
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_REQUEST_DATA {
     The HTTP request payload is buffered and accessible here
     Now use string functions to get the username from the request payload
    set username [findstr [HTTP::payload] "" 10 ""]
    switch $username {
        "bob" { pool bob_pool }
        "alice" { pool alice_pool }
        "eve" { pool eve_pool }
        "oscar" { pool oscar_pool }
        default { pool default_pool }
    }
}

Of course this is a wildly simplistic example, and you could a) use XML_CONTENT_BASED_ROUTING to parse the XML payload with real xpath processing, and b) use something more elaborate than a static switch statement to test usernames. In any case, the findstr command is a fast and easy way to selectively extract information from a string, in this case the data between "" and "".