>When held by a Pokémon, it increases the Sp. Defense stat by 50% but requires the Pokémon to only use damaging attacks.
Thats the effect of the assault vest.
Assault Vest is a great item for tanky offence. Examples include Tyranitar and Goodra, who have strong Special Defense in the first place, but with Assault Vest become even more bulky on the special side, and they both have excellent offensive stats, allowing them to hit hard and take hits.