Once filled and used, you do not want to drain an in-ground vinyl lined pool. There are many reasons not to drain the pool. The main reason is that over time the liner seems to become less pliable and when the pool is drained the liner contracts. When you refill the pool the liner does not expand to the previous shape and often rips. This is an expensive error. The other reason is that the weight and pressure of the water in the pool provides structural integrity, helping to prevent the walls from collapsing.