Login from external app without any redirect

[budget_ben]

I've tried to read the great variety of login/redirect threads on the board but haven't found anything akin to what I'm trying to do.

I have a flash app that accesses a wide variety of server-side data via php scripts. What I am attempting is to allow the user to 'log in' via the flash application residing on the same server. I put the 'log in' in quotes only because I'm not expecting the user to be able to browse the forums via my wee app, I just need to authenticate a username/password combo and acquire a valid session.

A brief rundown of the sequence of events:
1. user is playing with the flash app and would like to save his/her progress.
2. a dialog box prompts for the username and password.
3. I do a POST to https://mysite.com/handleLogin.php
4. the php page validates the provided data and returns a 'go/no go' string to the flash app.

The problem I am running into is that the login code below does not work without using the do_login_redirect() function. With the correct username and password, it will return 'true' for a success but I'm not really logged in - when I browse to the forum index.php it acts as if I have not logged in.

function validateLogin( $username, $password )
{
	global $vbulletin;
	
	$vbulletin->input->clean( $username, TYPE_STR );
	$vbulletin->input->clean( $password, TYPE_STR );
	
	$strikes = verify_strike_status($username);
	if ($strikes === false || $strikes >= 5)
	{
		return false; //locked out	
	}	

	if( !verify_authentication($username, $password, '', '', true, false) )
	{
		return $strikes + 1;	//fat-fingered the password?
	}
	else
	{
		//User and pw ok, let's log them in
		exec_unstrike_user($username);

		process_new_login('', true, '');
		
		//*****************************
		//	WHY MUST I REDIRECT?
		//*****************************
		//do_login_redirect();
		
		return true;	
	}
}

So now my questions (finally):
1. Does anyone know why there needs to be a redirect? Does it need one to properly set the cookie/active user list?
2. Is there a way around this?

I appreciate any help you folks can provide.