It's incredibly hard and inconsistent to get precision hits on Vex with a bow. I see a year old posts about this topic and it's still there. Bows are normal agains any other enemy but vex you just can't hit drits with a bow.
Vex has bit Wonkyness in the hitbox with other weapons also but bows are just unusable. Go test it out, it's redicillous.
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I basically main bows; they’re my most favorite weapon type in the game. I do not suffer the issue you have reported. For Goblins and Hobgoblins, aim for just a tad south of the juice box for the crit. For Harpies, aim for dead center of the red eye. Lead your shots correctly for distance and motion.