RAM is generally used to hold multiple things (such as programs) in memory.
If the device is only doing 1 thing, it does not need to keep a lot of things in memory, in fact for a device the the Vita, the only thing the RAM is used for is buffering.
SmartPhones don't need much because most people aren't doing much multitasking because they can only have 1 app open. When they switch to a different app, the values are written to actual memory to retain information. So, not much RAM is needed
Smart phones can too have multiple apps open, even my older Samsung Transform (android phone) supports multitasking. I have listened to music on it while doing other things like typing notes with it's little keyboard so it can be done. Some (maybe most) smart phones can't multitask but to say they all can't would be incorrect.
By the way, my Transform has less than 256MB of memory and also less than 256MB of flash storage yet still can do this.
Phones and gaming consoles don't need as much memory as computers to get stuff done because the programs/games for each use less memory for a few reasons and their operating systems use much less memory than windows does.
For example, Android is based on linux (several other mobile and non-mobile use versions of linux too) and linux usually is much less dependent on large amounts of RAM than Windows. It has less bloat and is usually coded more efficiently but also has less features allowing it to use so little RAM.
Console programs are also coded more efficiently and don't have the wider array of features that computer programs tend to have.
Gaming consoles can get away with using even less memory than some smart phones because they use their memory almost purely for the game or program they have running and I don't think any gaming console supports any multitasking so pretty much all of its memory goes into the one thing it is doing.
Also, yet again we have the removal of bloat and more efficiently coded programs than most/many Windows programs.