Vegas cant conveniently be used with an external mixer , because of its lack of direct outs, or dedicated mono ouputs. Busses in vegas are treated as stereo, so...ok an example.

Lets say you have your kick on track 1 and your snare on track 2, and you want them to appear respectively on channel 1 and 2 of your external console. In order to do this you must create a buss and assign both of these tracks to it. Then you must pan the kick HARD left and the snare HARD right. Then drop the level of both tracks by 6dB

If either or both of these tracks is also assigned to an assignable FX send in vegas, it is now the victim of the pan setting and especially for reverbs, the imaging is out the window. Also the level adjustment of -6dB you made will play funny as well. You could use a plugin like gpan to pan that send back to the center of the FX.

You can "double buss" as in assign two separate busses to the same hardware output. The first buss you would turn down to -infinity the right side and on the second buss you would turn down to -infinity the left side. Then you could assign the kick to buss one and the snare to buss 2. Thereby eliminating the need for panning or level adjustment.

However, neither of these is a very fun way to go, and in the second case, adding additioinal busses adds to sketchiness of the app and severely decreases its stability

I HAVE mixed a few albums from vegas summed on a console, by going a third route and just wasting every other channel and assigning EACH track to a separate stereo pair. This is a terrible waste of resources and system stability but itll work.

If all you are using vegas for is a digital razorblade and using per track insert FX, you can summ on a console just fine using the first panning + drop 6dB method and come out just fine, but you lose all the internal fx and tricks

From here on out this is only my opinions. A mackie 1604 is NOT going to give you he summing clarity you would want. Its noise and sound character also will wreck the benefits of " glue" that analog summing can offer. But it MIGHT sound better in the end just because of the easier and more immediate action of eq's and the ability to use outboard gear much more easily. For this route I'd go for something more like the Dangerous 2 Buss, a much cleaner and simpler path, for summing only. I find projects I mix in analog to be "mushier" than internal vegas renders, though sometimes I have a little more control, and the mushiness is traded off for much more of the specific sound I wanted, but then Im bummed for the lack of automation available inside vegas.

The external control surafaces available to most audio apps dont offer any summing, nor would you want them to. What they do is something much more important. IF youve ever tried to set the level, BETWEEN two guitar tracks for instance, with a mouse, having control of two faders simultaneously will give you soooo much more power and vision over the final mix, that it completely trumps any minor differences in signal chain quality. Ill take the ability to control my mix, tracked with mackie preamps, then mouse mixing with a set of 9098's ANY day.

If vegas 5 supports external control surfaces, we can have both! The future is looking bright