I need help creating an IRULE that redirect if pool members are down and url ends with /VCS

something like this should do that. I'm not convinced this would be the best way to deal with the condition, but it satisfy your stated goal, I think.

if{ [string tolower [HTTP::path]] starts_with "/vcs" }  {
   Assume the worst
  set CATASTROPHE 1
  foreach { pmem } [members -list yourpool] {
   scan $pmem {%[^ ] %s} ip port
   if { [LB::status pool yourpool member $ip $port] eq "up" } {
       set CATASTROPHE 0 
   }
  }
  if { $CATASTROPHE } {
     HTTP::respond 302 Location "http://www.domain.org"
  } else {
     Catastrophe averted!
  }
}

That's a redirect method. Another option would be just to select another backend utilizing a pool command, but that would be better handled utilizing priority groups and avoiding an iRule all together.

If no pool members are up, LB_FAILED will be triggered. It can be triggered for other reasons, but perhaps you wish to redirect any time that the traffic cannot be forwarded because of a failure? If so:

when HTTP_REQUEST {
    if { [HTTP::path] ends_with "/VCS" } {
        set vcs 1
    }
    else {
        set vcs 0
    }
}


when LB_FAILED {
    if { $vcs } {
        HTTP::redirect "http://www.site.com"
    }
}

The variable

$vcs