The technical bulletin Formation of Wrinkles in Installed Swimming Pool Liners refers to studies made by FMC and Union Carbide showing that though low pH is more detrimental to vinyl liners than other factors, that CYA can help prevent wrinkles as well. This may be due to the lower active chlorine level when CYA is present as well as CYA's pH buffering capability. For the brominated pools, the CYA shouldn't be lowering the active bromine level, but would still have some pH buffering capability. There might be some other chemistry going on more directly with CYA and vinyl as well.
Yet one more reason to have some CYA in the water even for indoor vinyl pools.