I don't know anything about the actual limitations SFAS but for me the full version Sound Forge is indispensible. I couldn't imagine doing serious work without it (or some similar editor).
I use Vegas for video editing. I was just wondering if adding SF Audio Studio would enhance the audio part of Vegas or would I be only dublicating Vegas' audio functions.
You'd not only be duplicating, you'd be taking a step backwards. The full version of Vegas has more features. The "studio" line is similar to a "lite" version.
I'm glad you posted your question though because it highlights a major criticism the audio only folks have about Vegas's lack of clarity in its audio positioning in the market place.