So I am not sure what the resoning was behind a pool this size with a liner. You already went to the expense of concrete why not now finish like a real pool and forget the liner idea.

I've seen vinyl pool builders pour concrete walls. I feel it's a waste of time when galvanized pool kits have been around for over 50 years. It's a faster process and probably a lot cheaper than pouring concrete walls.

The liner will stay in place in those areas provided a "bead" lock strip is used. It's a wedge shaped plastic strip that goes between the liner bead at the top and the bead receptor under the coping. The coping may be made of aluminum and it has a bead receptor built in.

Those areas will always have tension with the propensity of popping out of the receptor.