For starters, the Vita:
An exceptionally powerful system, in every way superior to all of it's competition and can do most of what a PS3 can do, and that is high, high praise for a handheld. The display is stunning, I want my next TV to be OLED, because it's that incredible. The joysticks improve game play over the PSP and the multiple touchscreens at first seem gimmicky, but with use are easy to love. Really the biggest issue that the Vita faces is whether or not games producers will continue to buy in. With smart phones being capable of so much, a handheld that does gaming and a few apps doesn't look to be as appealing against an iPhone or Andriod. However, the quality of the game play offered on the Vita is light years ahead of any phone on the market. Any real gamer will know the difference instantly.
Call of Duty on the other hand is quite a let down. The graphics are great but gameplay is too compromised, the missions too short and doesn't have zombies. Get the Lego: Batman 2 edition or Assassin's Creed for a much more positive experience.
On a side note: The Vita will need a bigger chip than 4GB if you plan on downloading games. It's 1-2 fullsize games per chip.