Given that the audio tools within Sony Vegas Pro are so good (I'm using Pro 10) -
is it at all possible to use an external audio mixer with a Firewire connection to control the faders within Vegas?
Rather than use Automation on every track, I'd like to be able to do everything in real time externally like with ProTools...

Yes.

Vegas will track faders from the "Mackie Control" surface, and perhaps some others. I have an inexpensive Behringer BCF2000 that emulates the MC, it works.

For me, it was a little finicky to set up the BCF, lots of unintuitive driver setups. Eventually, Vegas sees it as a MIDI device, perhaps the whole thing would go quicker for someone more familiar with MIDI.

Once setup, there is a preferences tab in Vegas for external control.

Once communication is established, you click a control in the individual track headers to initiate an automation mode. This allows you to write the volume envelope with the hardware fader, and, yes, the fader on the control surface will "fly" in response to the volume envelope. There is also an "envelope thinning" function, that turns out to be pretty important, because you can quickly write 50 or more points in a few seconds; this is difficult to deal with if you want to touch things up. Once thinned, you might have 5 points to deal with - much better.

It is also possible to control Pan, Solo, Mute, Arm Track, Record, Play, Loop, Stop, FF, REW, drop a Marker, etc. with various buttons on the control surface. I'd guess you could also do some custom mappings, but I never tried.

8 faders. I think there is a shift function to address tracks 9-16, but I never needed it. My sense was that if you devoted some time to it, it could really improve efficiency for audio overall, but especially in a tracking session.

EDIT: there's a bit more info in this thread, http://www.dvinfo.net/forum/what-happens-vegas/106864-tascam-fw1082-behringer-bcf2000-vegas.html , including links to a sony whitepaper on BCF control of Vegas.

Seth,
Thank you so very much. That is exactly the information I needed.