I'd say it's the matter of playstyle. Vectis's hit-scan projectile speed makes it easier to score headshot and imo that makes it more viable. Damage per second or damage per shot really doesn't matter, because hits/headshots are not guaranteed all the time. They are just easier with Vectis.
Also, Vectis has Armor Piercing damage, which scales better while fighting high-level enemies.