After my nose bridge took a beating the last several months while wearing my glasses in 'The Rift', I was a little skeptical after reading a LOT of forum posts with poor reviews regarding VR Lens Labs prescription inserts. One post had a link to a Thingiverse print (http://www.thingiverse.com/thing:1602460) of a adapter frame suitable for Zenni Optical lenses...I downloaded the 43mm frame file and sent it off to a semi-local 'printer' who did a fantastic job -- and ordered up the glasses from Zenni -- #550021.
FOUR days after ordering both, I've assembled and installed vision correction for my Rift. It went flawlessly, for less than 65 bucks total. I have moderately strong myopia, plus bifocals for everyday -- the adapter with lenses (single vision) inserted fit with about 1mm to spare behind the Oculus' lenses.
I went with Zenni, but the entire operation wasn't flawless. I made every mistake in the book.
Got the lenses mixed up and flipped around. The first problem, is that when you put the lenses in the adapter, you're may have the adapter flipped around, so you may be putting the left lens in the right adapter socket. Also, this will also reverse the curvature of the lens. If you make this mistake, you will notice the lens glass is hitting the headset glass, and the adapter won't snap in fully either.
At the very beginning, they tell you to mark the top of the lens, because these lens are oval-shaped, which makes it very easy to lose your orientation. I did this, but after I put the lens in, I noticed my vision was screwed up. After further inspection, I came to the conclusion that Zenni Optical put my lenses in the frame wrong! The left lens was put in 45 degrees in the wrong orientation.
I have a medium-strong prescription, so I can tell the orientation of the lens by noticing the glass is thinner near the nose, and gets thicker as it expands from that point. Also, one of my eyes has a stronger magnification than the other, so I can tell the left lens from the right by noticing the overall thickness of each lens.
Anyway, after much trial and error, in the end, it worked.