Too bad that niether worked. I f the vitamin C didn't work, then I am thinking it is an organic stain. I don't know about the diluted acid on a vinyl liner. I don't have a vinyl liner pool. Have you tried some diluted bleach? Maybe spraying some on the stain, then using a small scrub brush would work ,it should if it is organic. If when the level of the water was raised, it took off the ring, then I would think bleach would work. I know that there are pool cleaners that break down enzymes, like body oils and such, but I have not used them either. Maybe some others can say what they use.

As for the sequestering agent - you can buy it at any pool store. The directions are on the back, and it tells you what you need for a maintanence dose. You will be able to tell if you are using enough when you have no more "green hair". I would also read the trichclor puck container and see if they are the source of copper. Last year when I was buying them, I had a heck of a time finding the pucks without the copper - it seems a lot of companies put copper in them to act as an algacide., Too bad it only causes problems.