The Vyper2 allows two nitrox mixes, while the Vyper only allows one. The Vyper2 has the compass, but you have to hold it quite level, so it's not really a great substitute if you're doing navigation. (It's fine if you're turned around and just wondering whether the bow is left or right.) The Vyper2's display is nicer (it's basically a dot-matrix display, instead of the calculator-style display on the Vyper).
One thing I found out this weekend was that the Vyper will not allow you to switch from AIR mode to NITROX mode during a dive series (you have to wait until the no-fly time has expired). The Vyper2 (like the Gekko and other Suunto computers) *will* allow you to switch *to* NITROX mode from air mode during a dive series. If you leave them both in NITROX mode and just set them to 21% O2 when you're diving air, that's not too big a deal. Also, the Vyper2 will retain its nitrox settings until you change them, while the Vyper will reset back to 21% after a couple hours. You should *always* verify mode and (if in NITROX mode) O2 settings before each dive, but that's a difference in behavior between the models.
Personally, I really like the Vyper2, and if you're looking at getting one or the other, I'd definitely go with it. (The features could come in handy, and I like the interface updates in the display and menus.) If you already had a Vyper, I probably wouldn't recommend an upgrade to the Vyper2 (except perhaps if you wanted to dive two gases, but then you might be more interested in a deco-friendly "tech" computer, anyway).