im a pool technician and i have been for 11 years, and for about 9 years my main area of knowledge is vinyl liners. ive read what you have posted and there are a couple things that might help.
1- do you have a french well, or sump well? this is weeping tile place around the perimeter of the pool, having this installed collects al your ground water before it gets to your liner, which can b e pumped out using a submersible pump.
2- is your backyard a lower grade or properties around you sloping towards yours?
3-your pool liner may have a leak. get a diver to come in or a pool guy to do a drain and search to patch the liner from behind any questions you can contact me at email@example.com
hope this helps