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Calling one MXML from AS code
<mx:Button id="myButton" label="My Button" click="myButton_clickHandler(event)"/>

function myButton_clickHandler(event:Event):void {
  var child:Child = new Child();
  myVBox.addChild(child);
}

<fx:Script>
 <![CDATA[
    private function changeState():void {
        currentState = (currentState == "default") ? "showVBox" : "default";
    }
 ]]>
</fx:Script>
<s:states>
  <s:State name="default"  />
  <s:State name="showVBox" />
</s:states>

<mx:VBox id="myVBox" includeIn="showVBox" />
<mx:Button label="Show/Hide VBox" click="changeState()" />