I think it should be safe and its always better to cover sides, back and top (for privacy esp).
If you meant to use it on the inside of the tank, be sure NOT to use any tape or adhesive. And you should probably seal all seams, especially along the bottom. The snakes like to do their business in corners and you don't want that seeping behind any walls you create.
Make sure to use products that don't gass off noxious fumes, air out well before using
Thanks zina ill keep all that in mind im still a little hesitant but will see the polystyrene is a foam board at lowes but thats what 3d backgrounds are made of and the vinyl seems fine just worried about the finish