Probably this. If Viva is the same as the Prima, then she doesn't carry tenders so her only choice is docking. If she can't, and there are no local tenders, the Prima Class have to go elsewhere.
Although I have to say NCL is very poor at communicating changes, for whatever reason they happen. I had one trip where the shore excursion company notified me of changes a week before boarding, while NCL only admitted the change two days before docking in a note to cabins. That's deliberate deception.