TIWAppForm is the base class for all forms used in App Mode.
VFI it is not supported because of complexities in the VCL, and differing implementations between IDE versions. Some users report that it works fine for them. We have repeatedly tested it in Delphi 2010 and found it to be problematic.
Even in VCL forms applications, VFI has many issues that have caused many developers to avoid it. A common problem occurs when making changes to the base form. These changes often cause serious conflicts in the inherited DFM and make the base form de facto read only.
Instead of VFI, consider using frames.