Hi James,

As I understand, you create your drawings in AI, then export them to DXF. Am I correct?
If yes, then you need to adjust settings in AI, because the second step - opening document exported from AI in AutoCAD, then resaving it again - will not give you anything other than adding additional distortion to the geometry of the curves.
AutoCAD itself always produce correct smooth curves in the document regardless settings you make to screen curves resolution.

If you I am correct, and you create your drawings in AI - Can you share an example of the drawing (or part of it) here in AI format + the final result you get for Vendor B in DXF format? I'll look at it and will try to figure out the problem.