my pool liner has a tear. what can mend until i can afford liner replacement?

How  long is the tear and what shape?  If it's a straight crack, they do sell vinyl liner repair kits that can work both in the water and under water, such as this one:

(there are more, I just picked one with best reviews)  which has enough vinyl patch material to repair 500 square inches of liner.  I don't believe 4 oz glue bottle will last through 500 sq. inch of application but it will still get you plenty coverage. The vinyl strip in this kit is about 80 feet long, so should be enough for any repair that does not yet justify a complete liner replacement.

There are tricks to using the patches successfully, especially under water. You need to rub the sides of the tear clean with a rug. Then cut a patch that's longer than the tear by 2-3". Unless it's a very small tear, I'd use the entire width of the vinyl patch material. Apply the glue and fold the patch material, glue side in, onto itself. Then go (dive?) to the tear, unfold a small part of the patch and stick it to liner 2" below the tear and go up (or along the tear), keeping to unfold more patch as you move along. The idea is to avoid exposing the glue to water for too long. Once you've applied the entire patch, remove air and water bubbles from under the patch by rubbing them out towards the edges. If the tear is not too long, apply pressure to the patch with your hands on the entire patch. Otherwise try to apply pressure at the beginning and at the end of the patch. Hold for 30 seconds or more if you can. 

Don't disturb the patch for 24 hours (do not use the pool). It will completely cure in about a week.

This process appears to be a very economical process but there is however another water heater which is much more efficient. The device employs the strength of the sunshine in order to heat the swimming pool. Not only is a swim season prolonged, your pool heat expense drastically drops, occasionally disappearing wholly. Slap a few energy cells on the roof and start taking whole benefit of all those daylight hours.