You need to add the addedEffect to the GoodAnswer-item, not the VGroup.
Assuming that GoodAnswer extends a class that has the "addedEffect" style, example of Fade from 0 to 1 with duration of 2 seconds
<fx:Declarations>
<s:Fade id="fade" duration="2000" alphaFrom="0" alphaTo="1" />
</fx:Declarations>
<s:VGroup id="answersGroup" width="100%" height="100%">
protected function button1_clickHandler(event:MouseEvent):void {
for (var i:int = 0;i<3;i++) {
var good:GoodAnswer = new GoodAnswer();
good.setStyle("addedEffect", fade);
answersGroup.addElement(good);
}
}