as I understood it, having the grape halved but connected, or two grapes, touching, forms a dipole antenna of roughly the correct wavelength, making the grape a 'lightning rod' of sorts for the rf energy. less than ideal grape configurations result in less energy collection, slower heating of the grape rather than instant vaporization of its juices, and therefore no plasma.
furthermore, placing an inverted cup all by itself in the microwave, with nothing but air under it does not result in the plasma forming ... smoke particles from burning need to be present, either from a grape being vaporized or a match or whatever
thats my analysis at least