Why are vector quantizers not used in video codecs? Using a vector quantizer (VQ), groups of image samples (or motion compensated and/or transformed data) can be quantized as vectors. I'm curious about why VQs are not preferred in the modern, highly popular video codecs such as H.264/AVC and H.265/HEVC. A quick Wikipedia search showed that there are some codecs (such as Daala) using VQs. However, the best codecs, in terms of compression efficiency, do not use them. Is that because of the difficulty of designing a good codebook and efficient searching of the codebook?
Theoretically, VQ is always better than scalar quantization (SQ). However, as you already mentioned, the complexity of VQ is higher than the complexity of SQ. It increases exponentially together with the dimensionality of vectors. Therefore, the question is how to design a practical coding system which can provide a best rate-distortion performance under a given complexity constraint.

A designing of a codebook is another very important issue in VQ. The main practical concerns about using a codebook in VQ are:


How to generate a universal codebook for a large number of images.
How to scale the codebook for various rate-distortion requirements.
How much storage space is reasonable for storing the codebook in memory.


All this practical constraints make it difficult for VQ to force through standardization process.