Constant quantizer is inefficient in x264, why would it be more efficient than 2-pass in VP8/9? Even the official breakdown of parameters for VP8 states that 2pass is better.
I said lack of bframes is no problem, because comparing VP8's pframe with x264's bframe is only going to hurt x264. When VP8/9 still looks bad there, the bias clearly wasn't enough to skew the result
BTW VP8/9 doesn'T seem to have more stable quality in the individual frames, despite no bframes. It has this crap-soso-okay-soso-crap-soso-crap frame cycling, so for all practical purposes, I don'T care if it has bframes under the hood or not.
In any case, the results were fairly conclusive over a series of frames. I didn't check just one pair, naturally.
I would love if it was possible to use some psychovisual model in the encoder, but its --tune parameter has only two options: PSNR and SSIM. So not much hope there.