Magnet is the best idea, you can also just silicone them to the wall. Silicone is easy to remove with a razorblades when you want to move it.
You could also use velcro that comes in little packs where you peel the back of each piece off and it is sticky. Stick one side to the glass, one to the vine, and voila.
I would say suction cups are the worst option as they often fail especially with the constant temperature and humidity changes in a chameleon cage.