Erica,

In my opinion, those do not appear to be metal stains. As far as I know, Ascorbic acid is generally only used for metal stains and sometimes calcium deposits. I believe this may be black algae or some residue from algae. You say you lowered the PH to 6.8, Why? That is a dangerous thing to do since most testers only test down to 6.8 so your PH may in fact be lower which is not good for your liner.

Also, post some additional numbers for FC (free chlorine), CC (combined chlorine), CYA , along with PH & ALK. and let's see if you have any thing that is chewing up the chlorine. I'll bet someone here more familiar with vinyl liners has seen this before.

Dave