The cause of ink flaking off the vinyl is low water PH. An acidic environment will weaken the bond by softening the coating and eventually the ink. The more acidic the environment the greater the likelihood of damage. The effect is cumulative and irreversible. Once this softening occurs, the coating and ink are susceptible to abrasion and flaking.
Always keep your pool at the recommended PH of 7.4 to 7.8. Deviation from these levels will adversely effect the performance of your liner.