Vinyl is more pliable than paper, and can be stretched a bit, showing waves or ripples much like fabric although to a lesser degree. Light vinyl backdrops straight on, versus at an angle, to avoid accentuating any waves. If your vinyl was inadvertantly creased when rolled up, leaving it down overnight can help, You can also try gently relaxing the crease with a warm hair dryer. Add clamps to the bottom hanging edge of the vinyl to act as counterweights, pulling the material taut.