Hey,
Virtual Vacancy is generally defined as vacating the seat in pursuit of a better seat in second round.For instance,in first round of counselling you get admission in a college but in order to get a better college if you can leave your seat in the previous college to get admission in second round of counselling in different college. It happens when there is chances to get upgraded seat in subsequent cutoffs. Nonetheless when your seat gets vacant it is given to other candidates.
Hope this helps.
Good luck.