I'm thinking this too: In the vasa recta, quite a bit of fluid has been filtered out, with protein remaining in the blood. High protein and low fluid leads to high viscosity.
As to me, viscosity is a property of fluid itself and shouldn't have any dependancy on vessel's diameter.
Various fluids will only react differently in different diameters...
But still not sure about the answer 🙄
I will repost it in NBDEAnswers and see what those guys think!