I don't have problems. I'd say I have medium/average sized hands.
I curl my middle finger, ring finger, and pinkie and have the PSVita rest on the middle finger. With it loose enough for me to easily use the rear touch.
I have my thumbs on the analogs (duh) and index finger on the shoulder buttons. I also press a bit with my hand so that my midddle finger is able to be loose enough to use the rear touch. I can also move it away completely without issue.
For touch stuff though, I put my thumb on the bottom of the PSVita where the plugin for the usb charger is. And my pinkie on the other side. The rest of my fingers on the top holding it. And then use my other hand (right) to use the touch screen. That's how I did it for the touch QTE in Uncharted.