I have a new redesigned version for sale on cults http://cults3d.com/en/game/ergonomic-grip-for-ps-vita-v2-pch-1000.
I've been working on this for a few months now, and while it's not perfect, I'm confident enough to proclaim it better than any other alternative money can buy... Or at least in terms of comfort ;)
The Vita will fit nice and snug and apart from being ever so slightly larger; the shape of the grip is basically identical to a 360 controller so if you like those then you'll probably like this. As the title suggests, it's been modelled to fit the original phat Vita, and whilst the silhouette of the newer slim model might be close enough to fit; the cradle is likely too deep. Anyone willing and capable is welcome to try and modify it though.
I've made some slight alterations since I took the pictures. Specifically to the retention mechanism (if you can call it that) as well as a few other minor things, and whilst I haven't actually tried printing it, the changes are fairly minor so I doubt there'd be any problems.
I don't want to discourage anyone from printing it but I think it's fair to say that it will require some post processing in order to get the most out of it. Naturally, you'll have to glue a few parts, and for the retention peaces to do their job, you'll also need some soft adhesive pads. If you're printing in PLA (like I did) then I highly recommend water-sanding once it's all put together, as the unprocessed surfaces really isn't very nice to grip. I sanded mine with grits 120, 240 400 and the result is smooth and appropriately coarse.
I'd love to hear from anyone who tried printing it :)
I recommend at least printing the top cradle part with a concentric top/bottom pattern as the concavity will turn out better this way (the one in the pictures was printed with the default line pattern)
I also recommend that you add a few extra bottom layers. If you don't then you'll end up with few empty layers in the base sections of the two major parts.
I'm probably the wrong person to ask but with 3 shells (1.2mm) the print could probably do just fine without infill.