The Core compiler from VB6 is indeed the same as was present in VBA (Visual basic for Applications)  through VBA 6.3 as used in ALL MS-Office products (except InfoPath, which uses the .net-based Visual Studio for Applications, or VSTA instead of VBA). VBA remained basically unchanged since the VB6 days until Office 2010 shipped with an upgraded VBA7. However the only change of note were those changes necessary to have VBA code compile and run in a 64-bit envoronment as well as the previous 32-bit OS environment.

As for replacing VBA with VSTA or any similar .Net-based alternatives? It's just not going to happen, probably ever. The primary reason is that in order to go with VSTA, the entire core of all the mainline office apps need to be rewritten in .Net-based code because anything less will involve MASSIVE performance loss due to the large # of times that the .Net/COM interop layer needs to be crossed when running .Net-based code against COM/ActiveX-based object models at the heart of the C++ codebase for the Office apps.