Even in terms of tankiness the chest isn't really superior to the weapon. You only gain a bit more VIT if you upgrade from the 180 chest to the 200 one instead of upgrading from Hive weapon to eso weapon. And the gain in def/mdef is laughable.

Weapon is the biggest upgrade and the first everyone should buy. The increase in DPS is real and you still gain VIT too. As a tank you're still a damage dealing job, hitting things 100% of the time, like a DPS. Buying the weapon benefits more to your entire party than buying the chestpiece.