Emulation is not illegal. ROMs are not illegal. But ROMs are copyrighted. The only legal way of possessing a ROM image is if you possess the computer or device containing said ROM and you have paid for said computer or device.
It does not matter if you owned the computer or device at some point in time but not anymore, it does not matter if you own a similar computer or device which uses the same ROM(s), and it does not matter if the copyright owner is defunct: if you do not possess the physical ROM, then you are not allowed to possess its ROM image.
Do not ask me how to extract a ROM. Do not ask me to provide you with ROMs. Do not ask me to help you to find ROMs online. Do not ask me to validate or verify ROMs. Simply put: don't.
More information on ROM image legalities can be found on the Mini vMac FAQ.