I just noticed the first update of the PS VITA game "Ridge Racer" is online and did some research on the new update system.
It is a bit upgraded but basicly the same as on PS3. Once a game tab is opened, the VITA downloads a PKG for this game tab. This includes update information, such as if an update if available. If thats the case, the game tab will show you a yellow arrow on the top of the screen which gives you information about the update available. This information is stored in XML's which are gathered via URL's like this example one:
Since there is a hash inside the link, it is not possible to lookup any game by it's ID, yet.
The URL to the PKG update file may be stored in the first PKG mentioned above, as it's not in this "changeinfo"-XML.
Example update PKG: