[Molecula]

Так не пойдет, нужно так:

<mx:ViewStack id="rightStack" height="100%" width="100%" change="onChange()">
         <mx:VBox id="first">
		<mx:Button click="doing()" />
	</mx:VBox>
	<mx:VBox id="second" >
		<mx:Label id="welcomeLabel" text="Hello"/>		
	</mx:VBox>
</mx:ViewStack>

+

private function doing():void{
	rightStack.selectedChild = second;
}

private function onChange():void{
	trace(rightStack.selectedChild);
	welcomeLabel.text = "Hello viewstack";
}