Codecs (such as H.264 or VP9) describe a bit stream, and how to decode the bit stream. They basically provide a kit of tools that can be be used by encoders.
The quality of video encoding is mainly due to the technical knowledge and artistry of the encoder manufacturer and how the use that took kit. I can show you great H.264 encoders and horrible H.264 encoders, but they both emit valid H.264 bit streams.
In particular, the biggest challenge is rate control. If you don't care about the details of a variable bit rate, almost anyone can write a great H.264 or VP9 encoder, with the bit rate jumping up and down all over the place. However if you expect a bit rate to be held within say +/- 100 kbps, only a few vendors have the expertise to make a more constant bit rate look good.
I'll also add that I've seen no good data that shows that VP9 encoders perform better over a wide range of content than H.264.