Now, if we take as given, both in OTL and the ATL that the Vivaldi brothers had the idea that they could reach the Indies by sailing west around the world, could we take as the POD that ITTL the brothers become aware, as Columbus did 200 years later OTL, of the evidence that suggests there are indeed lands of some kind to the west--and that evidence mostly comes from the northlands? And therefore they plan their expedition to go by the northwest route, hiring local vessels and expertise along the way. Thus they take a viable route in viable ships, learning of intermediate stops they can make thus extending their range, and so reach North America and perhaps explore a considerable distance down the shore (after trying to penetrate northward and giving up in that direction, finding no useful Northwest Passage--so turning south to look for a Southwest Passage instead). If they are indeed to return, at some point they have to cut their explorations short and plan on going back by the known route they came on (which is tricky as currents that favored them going west will oppose them going east--but maybe some fishermen they hired know the alternate best eastbound route too).
Anyway, they know they haven't yet found China or the Indies themselves, but they aren't sure there isn't a westward passage to be found to reach them.
They probably have a more accurate idea of the size of the world than Columbus did, because he was a victim of wishful thinking and jumping to the conclusion that the western lands his Northern informants whispered or told tall tales about were in fact the Indies he wanted to reach--he therefore cherry-picked the range of opinion in scholarship until he found a theory that said the world was small enough, and Eurasia big enough, to match that notion. But long before Columbus, the erroneous calculations the small version of Earth was based on were discredited and corrected and most opinion settled on an Earth of about its actual size or even in some cases even bigger!
So the Vivaldis won't think they are in the Indies themselves and will be looking for passages there, not expecting to actually reach the Indies unless they are very lucky.
But then again, it was quite difficult to determine longitude, and they might be confused as to how far west they had actually gone.