Hi,
I wouldn't go your route by writing a VDD driver just overwriting the BIOS area.
The warning with the calling convention is OK for VDDs iirc, don't worry about it.
If you want to force NTVDM to use the *.rom files from System32-Directory, better patch config() function of NTVDM to set UseEmulationROM=TRUE.
Then also stub out the call to GetROMsMapped() so that NtAllocateVirtualMemory for the BIOS areas doesn't fail in the UseEmulationROM=TRUE branch in the same function and if you have luck, you can use v7vga.rom and emulated BIOS ROMs then, if this is what you desire.
PS: CPU isn't "emulated", it's running in V86 mode, so instructions are directly executed by Monitor on your CPU. Emulation is only there on non-x86 CPUs.