Probably the closest analogy to this would be holding a charged capacitor in a vacuum and asking what the potential between the capacitor and vacuum was. The thing is, (voltage/potential) is measured between areas of high and low electron charge. The vacuum is a chargeless or close to chargeless media, and it has no real way of retaining a charge. Vacuum capacitors are actually a thing used in high voltage scenarios because of how good of an insulator a vacuum is. Any charge you would measure would be between the positive and negative plates of the capacitor. If you pumped enough energy into the capacitor to start some sort of run off, well im pretty sure that would be equivalent to a cathode ray tube where instead of the vacuum retaining a charge, the electrons would shoot out until they hit something.