I would think that pva the brickwork before would be all that would be needed. If you are only making a repair moisture could still leach from surrounding plaster work though. but I would not think it would do any good to pva again before skimming.
You Will Need To Pva The Dried Browning, Or Any Other Surface That Will Become Porous Once It Has Dried Out, As You Will Have Found That The Plaster That You Have Put Onto The Dreid Out Bonding Probably Went Off Quick, The Problem With This Is Not That It Dries Too Quickly To Work With It But The Plaster In The Long Run Does Not Bond As It Should Because It Is Drying Too Fast
I Hope That This Helps