I have both. As already mentioned, entirely different knives. The small Subcom is a nice little forget-it's-there kind of knife, with great ergos. As much as I love the design, and as much as I carry it (more than the Vex), it isn't as utilitarian than the Vex. The Vex, due to the larger blade and handle, is more useful for most everyday cutting tasks. If you're just opening envelopes and such you're not going to notice much difference.
You get a lot more 'knife' with the Vex than the Subcom. It's built like a tank for a liner lock folding knife. And the fit and finish is quite nice. The only downside is that the clip is tip down only.