I don't have a Parhelia but this card has been discussed not so long ago so I'd suggest that while you're waiting for an answer, you could do a Search with "matrox" or "parhelia" as keywords. I think it's been a subject of discussion around last month. Questions were about DualHead features and all that "second monitor/ext. monitor/3 monitors" issue...
There's a good chance you have an answer when you get back to the forum anyway ! :)
P.S.: Yes, it's compatible with Vegas. And , by the way, I got this (now) "old" Millenium G450 and it works just fine so I figure out the Parhelia wouldn't be a bad choice at all, unless you're a gamer (or soon-to-be gamer)...
I have Vegas Video 4 + DVD and a Parhelia computer with a triplehead desktop. The machine I am using is dual-processor, also.
The benefits are:
1) really, really long timelines that I can use to make cool screenshots that make my friend Howard jealous :-)
2) combined with the dual-processor, I can give Vegas one of the screens at the side, kick off a long render, and keep the program up while I am doing other work, and I almost don't even notice. Plus I still have 2 screens left for email, web, compiling, etc...
I have one. In fact 3. Vegas doesn't have display drivers written for the Parhelia at this time. So, you only get the 3 monitors, or 2 monitors +NTSC, but the NTSC currently won't work in Vegas.