Check out this video by Mattias.
Its a bit old (I run Mac osx Lion 10.8.3) but the concept is the same for recording VST's in to Reason.
When i use soundflower to record audio, the resulting audio is still very low in volume but Mattius, in his video seems to be doing it with no probs?
Ill keep trying? i dont want to use my external audio interface lol