Mini vMac does not have a complex installer; it simply looks for the ROM in its immediate surroundings.

The ROM file for a Macintosh Plus is exactly 128 KB in size. If your file is 256 KB or 512 KB, you have selected the wrong target machine variant in Mini vMac.

If you tell me which operating system you are using (Windows, macOS, or Linux), I can give you the exact, step-by-step instructions to get your first system booted.

Look for pre-made, bootable .dsk images of System 6.0.8 or System 7.0.1.