First, you can use VAS as default microphone, then let VAS read your real microphone data. See:
http://www.virtualaudiostreaming.net/record-microphone-voice.html Second, I think Windows 8 should be able to change default microphone. If you click the button 'set VirtualAudioSteraming as the default sound record device' on the below screenshot, (The button is on the right panel of VAS), can you select your real mic in the pop up dialog?
Let me know your testing results. Thank you.