In a way couldn’t the Sap Mastery also be flavored in a similar way to this? You cut the person they have disadvantage on their Opportunity Attack similar to how the Speedy Feat works. You could even hit, run away to provoke the opportunity attack, then hit them again after they attack (Assuming...
I think the Sap and Slow Masteries fit here, as the Slasher feat is pretty much these two Masteries but slightly augmented. I think Vex makes it too similar to the Rapier mechanically. The Shortsword and Scimitar have different masteries which will affect the way a person will build their...
I think Sap and Slow fit here, as the Slasher feat is basically these Masteries but slightly augmented. I think Vex makes it too similar to the Rapier, as it would just be a damage type difference instead of being both a Mastery and Damage type difference.