Plagueis is substantially faster/more agile than Vader, and I can't see Vader getting the better of someone who trained their whole Sith career for the purpose of beating him. He takes down Vader.
Maul beating Venamis? If he does, it'll take longer than it takes for Plagueis to emerge victorious against Vader. Venamis is faster/more agile and can ambidextrously blend forms mid-combat/mid-air, and the fact that Tenebrous chose him to replace Plagueis says something.