To add to Marie's comments: 15ppm is the max you should add to a vinyl liner with NO CYA(stabilizer) or you risk fading the liner. But the higher levels of CYA likewise increase the ceiling for chlorine for your liner. If your CYA was over 100ppm, you could safely go to 25ppm of chlorine with no damage.

For the test kit. Specifically ask for a FAS-DPD chlorine test. DPD is NOT the same thing. I don't know if anyone but Ben or Taylor have them, but Leslie's web site sells Taylor chems and they have a single FAS-DPD test kit, just for chlorine.