If you are adding more than one inch of water to your pool per week, discounting splashed-out and backwash waste water, you probably have a leak. Do not allow leaks to go unchecked. Leaks can washout supporting back fill behind the walls, corrode the walls, and may wash away sand on the floor, creating large sinkholes.
It is not advised to drain your vinyl liner pool, or allow it to leak out below the level of the walls. The water in the pool holds the liner tightly against the walls and floor. If the water is removed, the liner must be reset with a vacuum to suck the liner into place while filling. Otherwise, large wrinkles may appear when filling a loose fitting liner. In addition, an empty liner pool may allow rain water to seep in under the walls, washing away and destroying the specifically contoured shape of the floor. There also exists the risk of a wall collapsing or caving in. Consult a professional for assistance in these areas.
Never add undiluted granular chemicals, specifically pH de-creaser and Calcium Hypochlorite (shock) directly to the pool. These particles will settle to the bottom, "bleach" the vinyl, and compromise it's strength and resiliency.
Resetting your vinyl liner: If the liner has been drained, or leaked out on it's own, it will need to be "sucked back" into place with a vacuum device to remove the air between the liner and the pool shell (walls/ floor).
The vacuum will be in place until the water level is at a predetermined point on the wall. The vacuum is then removed, and the pool continues to fill. This is necessary to ensure proper fit, and reduce or eliminate wrinkles in the vinyl. Prices will vary on labor and trip costs involved, but expect a few hundred dollars when it's all said and done.