You're looking for a helpful guide on the Mini VMac ROM!
If the ROM file is valid, you’ll hear a short beep, and the system will automatically begin to boot. mini vmac rom
Mini vMac specifically looks for a Macintosh Plus ROM (128 KB). A ROM from a Mac II or Quadra will not work with the default, stable version of Mini vMac. You're looking for a helpful guide on the Mini VMac ROM
When you open Mini vMac, it should automatically detect the file and boot to a screen showing a floppy disk with a blinking question mark (indicating it is ready for a system disk image). you’ll hear a short beep