Sounds like this might be a bug. Could you please post a small sample application that demonstrates the issue?
Here's a zip of a sample project: http://dl.dropbox.com/u/15760/LayoutTest.zip.
A workaround that I've found:
Switch to a BasicLayout while changing the itemRenderer, then apply the VerticalLayout afterwards.
Setting the layout after setting the itemrenderer should solve your problem. I can't explain why but it works.
Message was edited by: lavum