CAD tip # 9561:

' Get the DXF translator Add-In.  
Dim DXFAddIn As TranslatorAddIn  
DXFAddIn = ThisApplication.ApplicationAddIns.ItemById("{C24E3AC4-122E-11D5-8E91-0010B541CD80}")  
'Set a reference to the active document (the document to be published).  
Dim oDocument As Document  
oDocument = ThisApplication.ActiveDocument  
Dim oContext As TranslationContext  
oContext = ThisApplication.TransientObjects.CreateTranslationContext  
oContext.Type = IOMechanismEnum.kFileBrowseIOMechanism  
' Create a NameValueMap object  
Dim oOptions As NameValueMap  
oOptions = ThisApplication.TransientObjects.CreateNameValueMap  
' Create a DataMedium object  
Dim oDataMedium As DataMedium  
oDataMedium = ThisApplication.TransientObjects.CreateDataMedium  
' Check whether the translator has 'SaveCopyAs' options  
If DXFAddIn.HasSaveCopyAsOptions(oDocument, oContext, oOptions) Then  
Dim strIniFile As String  
strIniFile = "C:\temp\dxfout.ini"  
' Create the name-value that specifies the ini file to use.  
oOptions.Value("Export_Acad_IniFile") = strIniFile  
End If  
'Set the destination file name  
oDataMedium.FileName = ThisDoc.PathAndFileName(False) & ".dxf" 'same folder, or uncomment:  
'oDataMedium.FileName = "C:\myDXFfolder\" & ThisDoc.FileName(False) & ".dxf" 'fixed folder  
'Publish document.  
DXFAddIn.SaveCopyAs(oDocument, oContext, oOptions, oDataMedium)  
'Launch the dxf file in whatever application Windows is set to open this document type with  
i = MessageBox.Show("Preview the DXF file?", "Title",MessageBoxButtons.YesNo,MessageBoxIcon.Question)  
If i = vbYes Then ThisDoc.Launch(oDataMedium.FileName)  

In version 2014 see: