Assault Vest seems perfect for a bulky attacker. If it has high physical defense it can become a good mixed tank, and if it has high special defense it can become even bulkier on the special side. Also turning some checks into hard counters.
For example, I can imagine Snorlax, Metagross, Rhyperior, Tyranitar, Suicune, Slowbro, Escavalier, Latias, Latios, Meloetta, Conkeldurr, Gallade (Plus it has Drain Punch) Roserade with Giga-Drain possibly make a good use of it.
Not to mention that there are lots of utility attacks that also deal damage and so can still be used:
-Nuzzle to paralyze
-Charge Beam, Flame Charge, Power-Up Punch to boost stats
-Clear Smog and possibly Topsy-Turvy to haze
-Dragon Tail and Circle Throw to phaze
-Knock Off