Bypassing a VIP based on destination address

Thanks for the help Hamish... I've attempted to use the irule on the Performance L4 proxy_vip (Protocol TCP/FastL4 profile) but getting the following errors.

01070151:3: Rule [test] error: line 3: [command is not valid in current event context (CLIENT_ACCEPTED)] [HTTP::uri] line 3: [command is not valid in current event context (CLIENT_ACCEPTED)] [HTTP::header host] 

I'm using the code below which is exactly the same as what you suggested w/o the SNAT line. To be able to use any of the "HTTP::syntax", doesn't the vip have to be of type Performance (HTTP)??? Note that this is a proxy vip which is answering on TCP Port 3128.

when CLIENT_ACCEPTED  {

if { ([HTTP::uri] contains "webamil.server.com") || ([HTTP::header host] equals "webmail.server.com") } {
  use pool WEBMAIL_HTTP_POOL
  return
} 

use pool EDU_PROXY_POOL

}