5 hours ago, Celerity said:
Vulns in melee come from the damage noun, not the object material. If the material is fire but has a freezing bite damage noun, it will hit the ice vuln and nothing related to fire. The item's material is used for edges and those hit a material vuln.
The Solar Grasp is made of fire (of some kind, anyways), but does rays damage, which hits the light vuln, not fire.
Fire giants get their damage/proficiency bonus from the material.
You are right for the most part. Fire giants get bonus prof from the damage noun too i.e. if the weapon is made from steel but the damage noun is related to fire they will still get bonus prof (example flaming sceptre).