He likely means the belief that the holy see has been unoccupied since the death of Joseph Ratzinger - which is what Vigano seems to believe.

I suspect it's more a reference to +Vigano's Vitium Consensus argument for the vacancy ... except that Bishop Sanborn also holds a variation on that same position and endorsed +Vigano's statement on that particular topic.  +Vigano has hinted at questions about the pre-Bergoglian Conciliar papal claimants but just hasn't gone there ... yet.

In either case there's no need to single out +Vigano's opinion as something distinct.  If it's about vacancy since Ratzinger, then there's already a school of thought we here refer to facetiously as Bennyvacantism for short, and he'd by no means be its only or even primary representative.  And if it's about VC, he's not either the originator nor exclusive holder of that opinion either.

So either way it's wrong to single out his "school of thought" as if it were either exclusive to him or originated by him.