Firecat,
It really is highly recommended to include live plants in the chameleon habitat. Besides helping to raise and maintain the humidity of the enclosure there are the added benefits of improved air quality, lush hiding places and an available vegetarian snack for your veiled.
Although fear of poisonous vegetation is a real and understandable concern, there are (as mentioned) a variety of readily available plants that have been proven to be very safe.
Make sure any of these that you aquire are cleaned very well with soap and water and rinsed multiple times before including them in the environment. I would also recommend replacing the soil with a fertilizer/pesticide free soil and or covering the soil surface with screen and/or large river stones.
Veileds are attracted to soil and will consume it on purpose. perlite, vermiculite and larger pieces of bark or other "peaty" substance often found in soil could result in impaction so better to be overly cautious about this.
I think you will be pleased with the overall aesthetic of the addition of these plants.
-Brad