You’re suggesting that handhelds are only ever used outside of the home, which is not the case for everyone (me at least). Also you can tether Vita to your phone via Bluetooth for internet access.
Yes… well, not an Xbox because I have a PC. ;) But I would buy one once a critical mass of unique games I want to play arrives. I also have an Android phone, and an iPad. I realise I’m most likely an edge case…
I also must be unusual because I don’t game on the go at all, as I really don’t enjoy playing on the bus or when waiting in lines. I’ll just listen to music and stare at scenery instead. I don’t travel away from home for extended periods. In bed I’ll read books (on the iPad).
So almost all of my ‘mobile’ gaming is done on the couch or in my computer chair at home (or work). My choice of which device to use at any given time is not based on how far away I am from the big-screen games, but rather which game I feel like playing (or whatever has the freshest new-game smell). All of those platforms have some great games worth investing time in.
The least played devices are the phone/tablet, as they have the least compelling gaming experiences for my tastes. I do love some board, card and interactive novel games and generally play those there, but only because they are exclusive to the device. I find the phone/tablet awkward to hold and prefer physical, disconnected controls over a touchscreen. A cross-platform touch card game like Star Realms for example, I’ll play on PC since I don’t have to hold anything or zoom in to see the cards properly.
The most played device outside of my PC is the 3DS. That’s not because I’m travelling a lot, it’s because it has many excellent games that appeal to me that can’t be played elsewhere. I do wish Monster Hunter 4, Fire Emblem, SMT, Pokemon et al were on the PC, because I’d play them there if they were, but they’re not. And the 3DS is quite fun to hold and use anyway, with it’s touch/pen second screen and gimmicky 3D (drawing maps in Etrian for example gives me a buzz).
Same goes for the Vita. It has some great titles as well - I wouldn’t have been able to play Persona 4G without one. But again, once PSTV became available I bought one so that I could play most of those Vita games on a bigger screen with a PS4 controller.
So yeah, I’d just have a PC if everything were available on that platform. Since that’s not the case, and there are great games waiting to be played everywhere, whichever game I’m currently engrossed in gets the attention regardless of platform.