Here's your file converted to a solid by creating 2 planar surfaces to seal off the open areas and joining those all together.

This then can be exported into ViaCAD using SAT format, but ViaCAD will not shell it as-is I think because of the poorly formed surface corner that I show in the second set of screenshots above.

So you'll probably need to delete that particular surface and construct something better in that area and then try to go into ViaCAD after that.

Usually it's not very good to directly surface 4 curves where they do not form a distinct corner area (where 2 sides of the surface are smooth to one another). It makes for an degenerate surface normal in that spot and that messes up offsetting operations. So instead of trying to build a surface that has a corner like that you may need to build an extended surface that is better formed and then intersect it with the surrounding areas instead.

- Michael