When we say VAD, we're talking about Virtual Audio Device. Basically, it's any device labeled "Sonar game, chat, media," and so on.
These VADs show up once you fire up Sonar. You can tweak each one separately in the mixer inside Sonar, like changing their volume for each app or device.
If you want more in-depth info about Sonar, check out the related guides over here!
GLHF!