The acid from a Trichlor puck floating feeder that parks itself in one place is far worse for vinyl than bleach. However, it is true that you do not want to just dump in bleach or chlorinating liquid in one place quickly because it is denser than water and can settle to the bottom in pools without floor drains (such as most above-ground pools).

The solution to this is simple. Add the bleach slowly over a return flow (at the deep end if you have one) with the pump running and after you add it brush the side and bottom of the pool in that area to mix up the water. Once the chlorine is mixed in the water it is identical to that which comes from Trichlor -- it lasts just as long and has the same effect on the vinyl. Though the chlorine from Trichlor is the same, Trichor is very acidic and also increases the Cyanuric Acid (CYA) level.

He probably has experience seeing people use bleach or chlorinating liquid in vinyl pools where they quickly just toss it in and don't mix it.

Richard