URL Redirection with in another complex https to http redirection rule ? please guide me

You were pretty close. You don't need the pool command because you're not proxying the traffic. Here's what it might look like:

when HTTP_REQUEST {
    if { [string tolower [HTTP::host]] contains "something.com" } {
        HTTP::redirect "http://aredirect.something.com/Home.html?u=[HTTP::host][HTTP::uri]"  
    }
}

A request for "http://app.something.com/foo" should generate a redirect to "http://aredirect.something.com/Home.html?u=app.something.com/foo"

There is one thing though that may cause a problem. If the URL you're redirecting to is this same VIP, then you'll find yourself in an endless loop, as all URLs will contain "something.com".