URL client-side Redirect with URI intact

This hasn't been tested but roughly a framework for you to refine it. You would have to use this on the VS serving site2.com:

when HTTP_REQUEST { 
if { [HTTP::host] equals "site2.com" } { 
HTTP::header replace "Host" "site1.com"
virtual site1_Virtual-Server
return 
} 
}

This would have to go in the virtual server for site1.com:

when HTTP_RESPONSE {
if { [HTTP::header values Location] contains "site1.com" } {
HTTP::header replace Location [string map {"site1.com" "site2.com"} [HTTP::header value Location]]
}
}