I'm not sure veileds always have 80% humidity- if fact I really doubt it. There are published accounts of these lizards being found in areas where there is almost no vegetation or anything except the practically dead twig they are sitting on in areas that haven't rained in several years as well as the cooler, more lush and humid spots.
Just from looking at photos here on the forums of some of the more experienced keepers who choose to use glass- I also don't think most glass keepers provide as much room as most screen keepers. That is more a judgement of quality of life rather than needs of the animals.
I myself started breeding veileds and panthers and carpets and quads and montiums and jacksons 20 years ago in 29 gallon tall aquaria. They were healthy- some of the veileds were the healthiest you've ever seen even though they were by my standards today, pretty darn cramped. I used tanks because the breeder who introduced me to chameleons also used tanks that size.
But I was able to set up what they needed- thermo gradients, plants, etc. and they seemed pretty content. One of the forum members who boasts some of the longest lived veileds ever used(uses?) similar sized aquaria.
So it isn't what I'd call the "needs" of the animal when it comes to size of enclosure- if it finds what it wants it will be content in something smaller. Larger than what the animal is content with becomes a judgement call made by the keeper and has largely to do with imagination about what the lizard experiences as far as quality of life is concerned rather than what the animal actually needs (and most will seek to go through screen or glass occasionally- I'm talking about contentment most of the day most days).
But these days I much prefer 30x30x48" high enclosures for veileds, it is beautiful to see the chameleons stretch out and walk someplace, and I like to think I am providing a higher quality of life for the lizards. Some large male veileds will appear a little cramped even in a 24x24" rather than 30x30 to me nowadays- I have a large male that must be nearly 2' who is in a temporary 24x24x36 because I had the fire this year and my focus is on rebuilding my building before winter sets in before moving on to permanent cages and I wish he were in something a little larger. But he's fine in what he is in for now.
But that is why I use screen- cheap for size and can be kept outdoors. In my living room, I would have gone with glass and probably custum built something (maybe something clever with an open front that was still inescapable)- but I don't keep my chameleons there.
As far as glass enclosures not drying out and getting musty- this was never a problem in my tanks and I didn't even have side vents. If it is a problem in someone elses, I tend to think they are misting too much. I misted lightly morning and evening and ran a drip for 20-30 minutes which collected in a catch bowl. Puddles are bad and newspaper not drying out in a couple of hours and the tank not drying out completely between dampening means there is too much water being used. Everything should be dry within a couple of hours after misting or the end of the drip, except the water in the catch bowl. The drip, the water evaporating from the newspaper after mist, and the potted plants all raise the humidity enough without soaking everything several times a day. Even my mountain species did great on that misting and dripping schedule and did not have dampness problems. Maybe it is more challenging to balance things in a dry part of the country though.
Those are just my thoughts anyway.
Edit- and before this becomes a recommendation for 29 gallon tanks- I no longer use them and I don't recommend anyone else do so. Quality of life is an important consideration and my post is meant to balance the point of view that chameleons NEED huge enclosures, rather than a recommendation to use small ones. In his panther chameleon book, ferguson points out that in the labratory, enclosures as small as 10 gallon aquaria have worked for maintenance and reproduction of that species, but my opinion is just because it can work doesn't mean that is the target we should all be aiming for. Still there is a middle ground where rules are sort of guidelines and can give a little either way.
