Virtuos are pretty busy at the moment, they're doing the Switch versions of both Dark Souls Remastered and Starlink. You're right though, I would also expect that if/when GTA 5 gets ported to Switch (and come on, it really should, there is no good reason at all not to bring it over) Virtuos are the logical choice.
Virtuos is a big company from what I remember and looking at wiki they have 1,500 employees, so there is still plenty to go around if Rockstar wanted them. I doubt DS has a big team either, the game literally looked like a direct port. GTA5 will come imo, its just a when type of question.