I would consider this to be a bug within the Flex framework. If you would like to set the cornerRadius of a VBox, HBox, Canvas or most other container components, make sure you define the following:
borderStyle="solid" borderThickness="0" cornerRadius="20"
Example:
<mx:Vbox height="100" width="100" borderStyle="solid" borderThickness="0" cornerRadius="20"> </mx>