I have this surface:
This gives me these vertex normals:
However, I would like the vertex normals on the largest face to all be equal to its face normal like this:
Is there any way of doing this with Blender?
Thanks 
Why? What are you trying to get at?
Hey good question, wonder if there is a way to control vertex normal directions, I think I tried a addon a long time ago that re-normalized vertex normals but I cant remember exactly what it did, probably just adjusted them to the avg of other selected verts, but yeah this could be usefull.
I’ll post if I find anything
Edit: Well in the 5 minutes of searching that I did, I discovered that this is a long sought out feature, and that its only possible to adjust through scripting, but that when leaving and re-entering edit-mode the adjusted vertex normals are recalculated even if they were adjusted.
Maybe Bmesh fixed this, idk, but it would be nice to just simply be able to rotate a vertex normal, using r, or the widget or double tapping r. Which, I don’t think a addon could provide without creating a conflict with hot-keys already used. But if your really determined I think that it is possible with python.
if your normals seem skewed, ctrl A in object mode ( apply scale ) can fix them.
I can’t remember why this would be handy I just remember that I needed it once too lol