What are the audio production capabilities in Sony Vegas Pro because I see mostly plug-ins only?
If you are judging Vegas Pro audio editing on native effects than I don't think it has any edge over others.

I have been using the Pro version for about 18 months now and I find it pretty darn good overall. I am also learning Adobe Premiere and After Effects CS5.5, so far I think Vegas has the edge on the audio for sure.

However, are you aware that Vegas Pro audio editing has the ability to add as many audio tracks as your system can handle (stereo or mono), treat each track with a pan position feature, set individual volume (amplitude) envelopes for each clip, boost or buck the audio overall output of each track, name each track, size the vertical and horizontal view of the audio track as you see fit, record directly to any track, cut any audio clip any way you want, reverse an audio clip with two mouse clicks, and edit Dolby 5.1? And this is only a partial list of some of the built in controls available.

UPDATE: I omitted one feature that I really love, both audio and video tracks can have their order rearranged very easily. With a simple mouse click and drag, you can put that bass line near the drum track or move the voice-over above the music track. I like to move audio tracks as I am building them to keep it tiddy.
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This list is not complete and does not include the native sound effects which as mentioned earlier are rather standard with any NLE: reverb, EQ, pitch shift, delay, etc. And yes, AFAIK, it can accept plugins but I have never needed any at this point so I am not aware what plugins are compatible.

I have both Vegas Pro 10 and the home version of Sony Sound Forge. I can export an audio clip to Sound Forge for further effects, it's sort of like Audacity on steroids. As far as I know you can render your audio portion to several formats and export to anything that has the same format. I haven't tried this, but I would think you could easily export your audio as a .wav to Audacity but I have never needed to export the audio for further processing except to test it with Sound Forge, and in that exercise, I just had to copy paste the clip--easy.

I have made video content with Vegas Pro that has had as many as 18 stereo tracks with no issues.

Remember Vegas was originally a multi track audio editor.

I think the best answer is for you to try the trail version of Vegas Pro 11 for free for 30 days and kick the tires and then take a spin:

http://www.sonycreativesoftware.com/download/trials/vegaspro?keycode=64506

Disclaimer:
I should mention that I sort of have one hand tied behind my back on this answer as it would be so much more useful to launch Vegas when answering this to review the features. My version of Vegas lives on a machine that is not accessible from the network for security reasons and that location is about 8 miles from where I am typing this.