During the export process, on the "Export a VoiceAttack Profile" dialog (where you pick the location the file will be saved), you can use the "Save as type" dropdown to choose "VoiceAttack Profile Expanded as XML (*.vap)", as mentioned in the documentation (press F1 while VoiceAttack has focus to open VoiceAttackHelp.pdf in your default PDF viewer).
Note that modifying a .vap file and importing it is at your own risk. Making a backup of the original file is strongly recommended.