Persist with multiple cookies

In this case, you don't need the cookie persistence since you have the server information in the inserted cookie. By extracting this information in the request, you can send the connection directly to the server defined in the cookie. You don't need the virtual server name for your application, so I cleaned that up as well.

when HTTP_REQUEST {
  if { [HTTP::cookie exists "ltmcookie"] }{
    HTTP::cookie decrypt "ltmcookie" "ltmcookiepassword"
    set poolid [lindex [HTTP::cookie ltmcookie] 0] 
    set serverid [lindex [HTTP::cookie ltmcookie] 1]
    set portid [lindex [HTTP::cookie ltmcookie] 2]
    pool $poolid member $serverid $portid
    log local0. "Sending request to pool $poolid, member $serverid, port $portid"
  } else {
      pool pool_myhttp-pool
  }  
}
when HTTP_RESPONSE {
HTTP::cookie insert name ltmcookie value [LB::server] 
HTTP::cookie encrypt "ltmcookie" "ltmcookiepassword" 
}

Standard warning...untested (in this form, anyway!)

Updated to include a logging statement.