What is the risk of installing a pop up drain in a vessel with an overflow. "Apply only restrictively to those vessel sinks without overflow hole" What's that all about? Totally confused especially since the model had such.

Some vessel sinks have an overflow opening. Most do not. There are two types of drain stoppers. One for sinks with overflows and one for sinks with out. The one for an overflow will probably stop the drain but the openings in the drain for the overflow will probably show if the vessel sink is transparent.