It has always seems like a massive oversight that Sony didn't give the Vita L2/R2 buttons. Not only for perfect PSone compatibility, but for remote play for PS3 and PS4 as well.
I *hate* the rear touchscreen with a passion, so depending on the game, either map L2/R2 to the right stick or front touch pad. In some games that fine, in other games, it's a huge barrier.
Why oh why didn't they copy the Wii's Classic Controller, and make L2/R2 small buttons just in from the main shoulder buttons? It's not ideal, but a million times better than having to map them to touchscreens.