Vflex attribute on vertical tabbox cannot works

<window width="500px" height="500px" sizable="true" border="normal">
<tabbox orient="vertical" vflex="1" >
    <tabs>
	<tab label="One"/>
	<tab label="Two"/>
					
    </tabs>
    <tabpanels>
	<tabpanel>
            <groupbox hflex="1" vflex="1">
		<label value="One"/>
            </groupbox>
	</tabpanel>
	<tabpanel>
		<label value="Two"/>
	</tabpanel>				
    </tabpanels>
</tabbox>
</window>