This is really really strange.
I was trying to do the exact same thing only a couple of days ago!
(My friend's computer is broken, so I was streaming Guild Wars 2 to him over skype).
EDIT - Old Version
I think you'll need to have 2 physical audio devices.
I have a USB headset which also has a 3.5mm Jack input from my PC.
So I have a soundcard and also a USB soundcard. I just output everything from skype to the USB soundcard and left everything else.
So if you have any USB speakers, or a USB Headset, that might work. If not, you can get USB sound cards dirt cheap:
[href=http://www.amazon.co.uk/Trading-Virtual-Stereo-External-Sound/dp/B005NPK5DE/ref=sr_1_7?s=electronics&ie=UTF8&qid=1335813512&sr=1-7]http://www.amazon.co.uk/Trading-Virtual-Stereo-External-Sound/dp/B005NPK5DE/ref=sr_1_7?s=electronics&ie=UTF8&qid=1335813512&sr=1-7[/href]
Edit: I've come up with a better solution.
Create 2 virtual cables with VAC.
In Windows playback devices:
Set Line 1 as the default device
Set Line 2 as the default communications device
In XSplit:
Mute game sounds
In Settings, set the microphone as Line 1 (From VAC)
In Vent:
Set the output as Line 2
In VAC:
Output Your microphone to Line 1
Output Line 1 and Line 2 to your speakers
That should work.
