I use muriatic acid instead of dry acid--it's much cheaper and you don't have to use nearly as much to get the same effect. My vinyl pool is on its 7th season and I have no damage due to the use of muriatic. I pour it very slowly as far out from the edge as I can reach, and into the return stream so that the return stream dissipates the acid before it can sink to the bottom.

CAUTION:
Muriatic acid is very corrosive, obviously, and will do major damage if it comes into contact with your liner, or if you breathe the fumes. I do have a couple of small holes in my concrete deck where it splashed out of my container while being poured and didn't get washed off. I have in the past splashed it on my hand and it was no big deal, but I immediately immersed my hand into the pool to wash it off. I do always wear safety glasses, and I usually wear gloves while handling it. I will tell you from experience that if you get a whiff of the fumes it is very painful and feels as if you'll never get another breath again. Make sure that you and any other people or pets around you are upwind of the stuff while you're pouring it. To decrease the danger of causing splashes to your liner, you might try pouring your measured amount of acid into a bucket full of water and mixing before pouring it into your pool. (NOTE: ALWAYS add acid to water, not water into acid). If you're careful and respect the stuff, it is very effective and just as safe to use in a vinyl pool as dry acid.

Janet