I find it quite common for some of my chams to chew on some of the leaves from the live plants that are in their cages. I'd have to say mosty I catch them with ficus leaves hanging out of their mouth. However, grape vine bark probably isn't the best thing for them. Their is a good chance that if he gets a piece thats to big, it could cause a blockage. If I were you, I'd get that grape vine out of there and shave all of the bark that he could possibly chew on, off of the thing. Once all the bark is off I'm sure the grapevine would be a more than adequate perch for your cham. I hope this helps.