Hi Bruno!
I have been looking at the stats from a couple of years ago, from to today about the situation between IVAO vs VATSIM. And those two networks has almost every time I have been online been on the same amount of pilots.
But in the latest time, after people have installed Vista, I know there is a well known bug with FSInn and Vista with FS2004 on it. And at the same time many people, including my self, having problems with the IVAO program. And VATSIM has newly launched the new Squawkbox 4 version, which supports both FS2004 and FSX, and then pilots who had trouble with FSInn, comes back to VATSIM.
I'm not a witch, but I think you will see even more people on Vatsim in the future, and perhaps IVAO will have problems with the number of pilots because of that reasons.
I also know that many pilots from my old VA didnt like IVAO, because it's not so "free to flight" at IVAO. IVAO got their own system, with own tours and exams. You dont have to go through that system in Vatsim.
I know this may sound like I'm a big Vatsim fan, but it's not that, it's only a question on what programs they use, and what is compatible with what. Vista with FSInn isn't that stabil, and the IVAO program doens't work very well eighter.