Apparently Graybox has very little to do with Mini vMac, despite what the site suggests. It only uses a small bit of Mini vMac code.
The site just says that it "uses Mini vMac's M68K emulator", which is exactly what it does. (The emulation of the 68000 processor, which is a fairly small part of Mini vMac.) There is no other connection with Mini vMac. Also, GrayBox is not much like Executor. As the site says, it takes the same approach as Apple's "Classic" environment. It makes full use of the software from the older operating system, but extensively modifies it so that the old application is tightly integrated into the modern operating system.
Not much attentions has been paid to copyright law, but none the less it is impressive work. The copyright issues could be fixed by letting the user supply their own copy of the old application, ROM, and system software, as other emulators do.