Hi,
if i checkout, with a logged in user with just username and password in user data and i fill out invoice address and click add shipping address the invoice address is gone after saving shipping address.
i use VM demo theme but disabled template override for userdata. So it should be the original VM userdata checkout.
thanks
http://www.santegra.de/de/produkte
Please try the whole checkput progress.
I use the VM demo theme.