Please refer to Blacksmith Pushkin in the Town of Giran to exchange a common item of Vesper for a rare item.

To exchange an Upper Armor piece, you need: 4 S84-grade Spheres.
To exchange a Helmet and Lower Armor, you need: 3 S84-grade Spheres.
To exchange Gloves, Boots, Shields, and Sigils, you need: 2 S84-grade Spheres.
To exchange Accessory, you need: 1 S84-grade Sphere.
To exchange Weapons, you need: 5 S84-grade Spheres.