The Playstation Vita will be quite a powerful handheld gaming system, by all accounts, but there are some things it won't be able to do. One such thing is the ability to have multiple Playstation Network accounts linked to the same Vita. The PS3 allows several users to create their own accounts and toggle between users.
One other thing users won't be able to do is pause a game and toggle to the device's Web browser. This is another feature that the PS3 is able to do but alas will not be functional on the Vita.
The reason the Vita is being compared to the PS3 is because recently, it was revealed that the device will be able to have full-access "Remote Play" functionality. That is, you'll be able to tap into your PS3 from your Vita as long as both devices are connected to the Internet.
So if you're playing the latest God of War game on your PS3 and need to head over to your parents' house for dinner, you'll be able to continue the game from your Vita while sitting around and trying to avoid dinner table conversations.
Although there are some limitations, the Vita will go leaps and bounds beyond the PSP with such features as trophy support, pausing games to check friend lists, Twitter messages, etc, and the ability to connect to other mobile gamers on Sony's Playstation Android app.
The Vita will be released in the US in February. The PS3 was recently updated to make way for the impending connectivity features.