Well, considering his multitasking (like suppressing Vaylin's power) while stomping the Jedi team, with enough control and preciseness to knock them all out without killing them, and how he one-shotted Revan in a weaker form, I'd say his decisive superiority in the Force would grant him the victory here. It is very unlikely that Plagueis will get to use his advantage in lightsaber combat, especially since Vitiate can easily start casting force attacks before Plagueis closes the 20 meters gap, and Plagueis is an insanely knowledgable user of the Force, who thinks that lightsaber combat is a waste of time. I doubt he would ever engage someone attacking him with the force in any manner other than responding back with the Force.