Exporting Visio drawings to autocad dxf/dwg using vb6

I'm trying to save a visio drawing as an autocad DXF and/or DWG file from a VB6 front end using the Visio Oject Model.
The drawing itself is quite simple and built from the front end using methods such as PageObj.DrawRectangle().

If I use the format

PageObj.Export "myfile.dxf"

then a dialog box comes up asking which layers I want to export (for my file there is only one layer).
However, I would like to export the file with no operator intervention because visio is being
used as an intermediate step in a longer process.

Does anybody know how to do this?
Thanks for any help,
Chris.