Bleach and chlorinating liquid are denser than water so if you dump them quickly in one place with no circulation it can settle to the bottom which isn't good. That's why you want to pour slowly over a return flow with the pump running, preferably at the deep end, and then for extra safety, lightly brush the side and bottom of the pool to ensure thorough mixing. Once mixed, the chlorine is perfectly safe and gets well diluted.

My own pool is plaster and has floor drains so I don't bother with the brushing. It's probably overkill even for vinyl so long as you pour slowly into a return flow, but I give that advice just to play it safe since there used to be PBs that swore bleach was bad for vinyl -- haven't heard from them in a while and thousands of vinyl pool owners using bleach or chlorinating liquid and properly pouring haven't reported any problems.

Richard