Are these synonyms? Two different technologies? Can’t seem to google an answer.
I’ve always taken it as hardware driven vs app driven midi signal send and receive. But I’m probably wrong, end of the day I just wanna make music… 
Yes, Core Midi generally refers to using external hardware such as plugging a Midi keyboard in, while Virtual Mdi is app to app internally on an iPad (or whatever), so no need for any external hardware.