I know of an extensive list. Note that all users are best off as tanks/ bulky offensive.
T-Tar
Assault Vest + Sand sp. def boost is unbelievable, at 738 with max sp. def EVs. It's pretty much the ultimate Gengar stopper if you needed one, and I think most people do in Ubers. Pursuit trapping is going to be Assault Vest T-Tar's main attraction. T-Tar can survive non-STAB Focus Blasts and then deal out what damage it needs to with its wide movepool. The forfeit of Leftovers recovery will decrease T-Tar's time on the field, and that's the major drawback.
Heatran
The main things that Heatran checks are Sylveon, (Mega)Gardevoir, Alakazam, Lati@s and Talonflame, and with Assault Vest, Heatran will be able to survive a Focus Blast from Alakazam. It'll be a really great Fairy counter if your team needs one.
>With the EV spread 252 Sp. Atk/ 252 HP, Focus Blast from Life Orb Alakazam is 2HKO.
With the EV spread 252. Sp. Atk/ 252 Sp. Def, Focus Blast from Life Orb Alakazam is 99.6% 2HKO.
With the EV spread 252 HP/ 252 Sp. Def, Focus Blast from Life Orb Alakazam is 3HKO.
Again, Heatran misses Lefties just like T-Tar, so it's not for every team.
Metagross
Metagross usually runs a fully offensive set anyway, since it has Meteor Mash and therefore no need of a stat boosting move, making it a perfect Assault Vest user. It's got a wide movepool, and also a great STAB priority in Bullet Punch, which has seen much more use since Fairy was introduced. Metagross is still susceptible to burn though, but Assault Vest is well worth considering, especially since it packs Earthquake and is liable to be trapped by Magenezone's Magnet Pull.
Tangrowth
Kinda weird to have Tangrowth on the list, since it's a common user of Sleep Powder, but with Regenerator, Tangrowth can manage to be a mixed tank. It also has some pretty good offensive stats already, base 100 atk and 110 sp. atk, good movepool: Rock Slide, Knock Off, Brick Break, Earthquake, Giga Drain etc.
Tangrowth also checks a lot of stuff, like Azumarill, Starmie, Thundurus, Aegislash, Celebi, Rotom-W et. al, so it's a viable user of Assault Vest, especially with Regenerator.
Rotom-W
Considering Rotom-Wash only has 1 weakness, and Grass is a mostly special based move, Rotom-W is a good candidate for Assault Vest. It has a sparse movepool but at least its 2 STABs can cover most things except Grass, for which it can Volt Switch escape from, or carry HP Ice.
However, Rotom-W forfeits Lefties, Pain Split, Trick and Will-O-Wisp, so Assault Vest Rotom-W is probably not as useful as its support or Trick counterparts.
Conkeldurr
Access to Power-Up Punch, Drain Punch, Mach Punch and Guts makes Conkeldurr a really good Assault Vest user, since it doesn't need any boosting moves, and Guts means that it won't fear poison spikes or burn hax. The Sp. def boost will allow it to survive a STAB Psychic move from Alakazam:
>252 SpA Alakazam Psychic vs. 252 HP / 4 SpD Assault Vest Conkeldurr: 288-338 (69.5 - 81.6%) -- guaranteed 2HKO
and deal back the damage
>252+ Atk Conkeldurr Knock Off vs. 4 HP / 0 Def Alakazam: 308-364 (122.2 - 144.4%) -- guaranteed OHKO
Goodra
Great movepool plus an already monster sp. def makes Goodra a great Assault Vest tank. This just means that it has a a chance of standing against threats like Sylveon and Gardevoir. Mega-Gardevoir's more of a problem though.
>4 SpA Pixilate Sylveon Hyper Voice vs. 252 HP / 4 SpD Assault Vest Goodra: 132-156 (34.3 - 40.6%) -- guaranteed 3HKO
>252 SpA Pixilate Mega Gardevoir Hyper Voice vs. 252 HP / 4 SpD Assault Vest Goodra: 216-254 (56.2 - 66.1%) -- guaranteed 2HKO
>252+ SpA Goodra Sludge Wave vs. 4 HP / 0 SpD Mega Gardevoir: 158-186 (56.8 - 66.9%) -- guaranteed 2HKO
Rhyperior
Rhyperior's max sp. def in Sand with the Vest is 515, meaning, along with Solid Rock, it can take water and grass hits, which makes it a great tank. It's also got a wide movepool and a STAB Edge-Quake combo.
Some calculations...
>4 SpA Rotom-W Hydro Pump vs. 252 HP / 0 SpD Assault Vest Solid Rock Rhyperior in Sand: 270-318 (62.2 - 73.2%) -- guaranteed 2HKO
>252 SpA Rotom-W Hydro Pump vs. 252 HP / 4 SpD Assault Vest Solid Rock Rhyperior in Sand: 333-396 (76.7 - 91.2%) -- guaranteed 2HKO
>252 SpA Alakazam Focus Blast vs. 252 HP / 0 SpD Assault Vest Solid Rock Rhyperior in Sand: 145-172 (33.4 - 39.6%) -- guaranteed 3HKO
>252 SpA Alakazam Energy Ball vs. 252 HP / 0 SpD Assault Vest Solid Rock Rhyperior in Sand: 219-261 (50.4 - 60.1%) -- guaranteed 2HKO
Snorlax
Considering that Snorlax has monster sp. def anyway, giving it an Assault Vest would just make it unstoppable without a physical move. It's also a great Fairy counter to stuff like Gardevoir and Sylveon, considering that it can use Iron Head (tutored in Gen V).
>252+ Atk Snorlax Iron Head vs. 252 HP / 0 Def Sylveon: 242-286 (61.4 - 72.5%) -- guaranteed 2HKO after Leftovers recovery
>252 SpA Pixilate Sylveon Hyper Voice vs. 252 HP / 4 SpD Assault Vest Snorlax: 105-124 (20 - 23.6%) -- guaranteed 5HKO
Suck eggs Sylveon!
Magnezone
Magnezone's max sp. def with the Vest is 459, though realistically it'll be around 324 because all of its EVs and Nature will be geared towards its HP & sp. atk. Magnezone also has very few status moves to make use of, and would probably do well with: Charge Beam or Volt Switch/ HP Fire/ Thunderbolt/ Flash Cannon and Magnet Pull to trap stuff like Scizor. Watch out for Heatran though...
>252 SpA Alakazam Focus Blast vs. 252 HP / 4 SpD Assault Vest Magnezone: 196-232 (56.9 - 67.4%) -- guaranteed 2HKO
Lopunny & Swoobat
Strange users of the vest, but Klutz + Switcheroo/ Trick will be able to cripple your opponent's support/ cleric. Mostly NU users, but they're still good in crippling any Stealth Rock/ hazard setters, of which there are an abundance of in NU and a lack of spinners. Having said that, they serve pretty niche roles when holding Assault Vest, so maybe Flame Orb is better in order to ruin physical Pokemon.
Smogon Damage Calculator used
Smogon thread - Assault Vest
