Using URI to rewrite host

I'm not sure what I can say about the Javascript not working. Perhaps they have hard-coded some absolute uri's in there that are not based on the original requested uri.

As for the host, you could add a check to see if the token is already there and not add it if it is. Also, I'd move the setting of the uri/host into the if body so you are only changing it when it needs to be changed.

rule host_rewrite2 {
  when HTTP_REQUEST {
    set host [HTTP::host]
    log local0. $host
    set uri [HTTP::uri]
    set token [getfield [HTTP::uri] "/" 2] 
    if { $token eq "" } {
      log local0. "no path specified so ignore"
    } else {
      if { [matchclass $uri ends_with $::file_extensions] > 0 } {
        log local0. "Found file request, leave as is!"
      } else {
        log local0. "Not a file request, update host and check for trailing slash "
        if { not ( $host starts_with $token ) } {
          set host "$token.$host"
          log local0. "Updated host with token: $host"
          HTTP::header replace "Host" $host
        }
        if { $uri ends_with "/" } {
          log local0. "Uri ends with a slash, all's good"
        } else {
          log local0. "Uri doesn't end with a slash, appending one"
          append uri "/"
          HTTP::uri $uri
        }
      }
    }
    log local0. "host: $host"
    log local0. "uri : $uri"
  }
}