Well, the way I've been using it works quite well.
Adamant
200 HP / 174 Atk / 136 SpD
- Power Up Punch
- Seed Bomb
- Earthquake
- Rock Slide / Stone Edge
If you'd like some calcs just to show what types of hits he'd be able to take here ya go;
252 SpA Mega Gengar Dazzling Gleam vs. 200 HP / 136 SpD Assault Vest Chesnaught: 154-182 (41.9 - 49.5%)
252 SpA Volcarona Fiery Dance vs. 200 HP / 136 SpD Assault Vest Chesnaught: 194-230 (52.8 - 62.6%)
252 SpA Life Orb Alakazam Psychic vs. 200 HP / 136 SpD Assault Vest Chesnaught: 283-335 (77.1 - 91.2%)
Keep in mind, those are all SE hits. As long as you retaliate with the correct move;
176+ Atk Chesnaught Seed Bomb vs. 0 HP / 0 Def Alakazam: 220-261 (87.6 - 103.9%)
176+ Atk Chesnaught Stone Edge vs. 0 HP / 0 Def Volcarona: 560-660 (180 - 212.2%)
176+ Atk Chesnaught Earthquake vs. 0 HP / 0 Def Mega Gengar: 238-280 (91.1 - 107.2%)
It's been working extremely well for me. Perhaps because it's unexpected, but regardless, taking those hits, rather well might I add, and OHKO'ing for the most part is an awesome feeling.
The only thing that sucks when it comes to this is the lack of recovery, but, that's just the way Assault vest works. Maybe, if and when he gets Drain Punch, he'll be better off.