Use Aquarium silicone. you may also be able to use GE Silicone II. you can go to
www.garf.org/ click on DIY Pages and then on tank building. you put in the dimensions you want and They will give you info on thickness of glass overall gallons, a cut list, additional materials list and step by step instructions on how to put it togeather. A great site. also remember they are considering a tank that will be full of wtter so for a viv you can probably use glass that is of a lower thickness. for aquariums thickness depends on hight so for a tall viv they will tell you very thick glass. I say thick is good but if you are concerned about waight or cost do what you think is safe