I just ran across this at Leslie's website:
Muriatic acid is heavier than water and may etch the plaster in a shallow pool. To eliminate this possibility, pre-dilute the acid first by adding the acid into a large plastic bucket of water, and then pour the acid solution into the deep end of the pool, away from skimmer and fixtures. Muriatic acid is NOT recommended for vinyl-liner, painted or fiberglass pools.
Is this really a concern? I thought that acid would not hurt the vinyl.