I’ve read JCS’s book and listened to he and Bill Strand talk about it, and I tend to agree. Vitamin A derivatives are vital in eye health and function, and Chams have very high functioning vision.
It seems to me that they would benefit greatly in evolving the capability of converting carotenoid precursors to vitamin A.
It’s certain large species eat vertebrate prey and gain vitamin A from that, and it’s certainly possible that there are smaller insects endemic to chameleon’s natural range that may also have preformed A and thus they don’t need to convert.