You said..."He has UV light(natural and flouresent)"...neither should pass through glass or plastic.
What are the temperatures in his cage? Basking area? The rest of the cage?
You said..."I dust his crix with repti-cal and herpivite"....how often and how heavily?
You said...."Maybe the pet store isn't gut loading"...its something you need to take responsibility for. As Will indicated gutloading is something that needs to be continuous and with the intention of making the insect healthy and containing healthy things that will eventually end up in your chameleon. Here's a gutload that is recommended by many people...
http://adcham.com/html/husbandry/gutload.html
You said..."Lately he's been resting on the vines in his cage with his body resting on the vine like his legs aren't strong enough to hold him up"....one of the signs of MBD (metabolic bone disease) as stated by a vet is "a chameleon may show inability to get up on its legs and lift its body off the ground (called lack of "truncal lifting"
"....
http://adcham.com/html/veterinary/mbd-fractures-kramer.html
Have you had a fecal done on the chameleon?
Is he pooping? Do you have a substrate? What plants are in your chameleon's cage?
You said..."His legs also look a little skinny. He also seems to be sleeping alot during the day. Today one of his legs was hanging off of the vine"...all of these things are not signs of good health. I think you should get him to a vet ASAP. If your chameleon has MBD he needs help to get things back in balance and the husbandry needs to be corrected to prevent it from happening again, IMHO.
Have you had a fecal done on the chameleon?
Is he pooping? Do you have a substrate? What plants are in your chameleon's cage?
Hope you can get him healthy again.