Here is another method you can try. I know this process will export the sketch out to a DXF file, but whether or not the resulting file is formatted to AutoCAD 2000 is another issue. It uses the same command found within the right click menu when you select a PlanarSketch object in your model browser tree. It just bypasses the dialog box. One of the things in that dialog box, though, is the option to select an earlier version of AutoCAD you want the resulting file to be compatible with. This is a similar situation to using the SaveAs() method, because it too bypasses the usual dialog boxes, where you would normally be able to click on the Options button and set your settings. In theory, if you have already exported a DXF file the way you want this one to be, it should remember those settings the next time you attempt to export a DXF, and use those same settings. Or if you have a saved XML file for these settings, that you usually use, that file may already be selected, by default, because that's how you did it the last time you exported a DXF. It's not the most controlled and stable process, but it may work for your situation.
Here's the code I used to test it:
If ThisApplication.ActiveDocumentType <> DocumentTypeEnum.kPartDocumentObject Then
MsgBox("A Part Document must be active for this rule (" & iLogicVb.RuleName & ") to work. Exiting.",vbOKOnly+vbCritical, "WRONG DOCUMENT TYPE")
Exit Sub
End If
Dim oPDoc As PartDocument = ThisApplication.ActiveDocument
Dim oPDef As PartComponentDefinition = oPDoc.ComponentDefinition
Dim oSketch As PlanarSketch = oPDef.Sketches.Item(1)
Dim oFileName As String = "C:\Temp\Sketch.dxf"
Dim oSelSet As SelectSet = oPDoc.SelectSet
oSelSet.Clear
oSelSet.Select(oSketch)
ThisApplication.CommandManager.PostPrivateEvent(PrivateEventTypeEnum.kFileNameEvent, oFileName)
ThisApplication.CommandManager.ControlDefinitions.Item("GeomToDXFCommand").Execute2(True)
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Wesley Crihfield

(Not an Autodesk Employee)