Use a bigger size rock, not sure about veilds, but I use 3/4 - 1 inch growstone as partial substrate in my panther viv, and have not had any problems, if I were you I would remove the plumeria, it's toxic, I made that same mistake, veilds eat plants, I would do some heavy research on plants, real plants are much better than fake, your set up looks awesome, once you get it veiled safe your cham should be very happy!!!
I would never use pebbles, and I’d personally only use rocks to top off planters, if that. I use sheets of moss on the bottom of my cages over the drainage system or substrate. FL Chams has an extensive cham safe plant list