When we developed VSTA, one of the key scenarios for us was that VSTA should work side by side with VBA. We understand that developers would be unable to cut over cold to VSTA. Just like with COM interop VSTA and VBA side by side allows developers to migrate their VBA solutions over time when it makes business sense to do so. Summit has written an excellent article to demonstrate ShapeApp integrated with VSTA and VBA at the same time.
Integrating VSTA with an Unmanaged VBA-Enabled Application