It's never good to wait until your pool's vinyl liner is showing lots of tears and other such damage before you have that liner replaced. That liner does more than just keep the pool soft underfoot; it protects the pool's construction materials from potential water damage and also ensures there are no costly water leaks in the pool. Note a few signs to look for that tell you it's time to get the vinyl liner replaced in your pool, so you know it's always in good repair and your pool is always protected.
Fading and stains
You can clean some algae and other debris that causes staining of the vinyl liner in a pool, but rust stains often cannot be removed; many stains are also a sign of deterioration of the liner and may actually be holes and damage, not actual stains. If you've scrubbed the liner and are maintaining the pool chemicals as you should, but there are still stains on its surface, it's probably time to have the liner outright replaced.
Slipping and stretching
Vinyl liners will eventually slip out of place and stretch away from the area around the pump and filter; when this happens, they can often be pulled back into position somewhat easily. However, once you notice a lot of bunching up around the bottom of the pool and along the sides, and you cannot pull or smooth out this bunching, the liner may need to be replaced. As vinyl liners get stretched over the years, from being walked on and from the weight of the water, they may eventually lose their shape and cannot stay secure around the surface of the pool. When this happens, your pool is at risk for water leaks, and the liner should be replaced.
The water levels of your pool can become very low for a number of reasons; during the hottest days of summer, water can simply evaporate, or a big pool party can mean lots of splashing and water displacement. However, consistent water leaks, or your pool losing water for reasons you can't otherwise explain, often mean that the vinyl liner has become damaged and needs replacing. You may be able to find small holes and repair them with a simple repair kit, but larger holes and tears are often too big to simply patch over. To avoid continuous loss of water and resultant damage to your pool and its base, plus the expense of having to replace that water, have the liner replaced.