I'm new to competitive battling but I want to quickly drop a couple of ideas for assault vest users here. I've tried Carracosta with decent success, though haven't tried Ludicolo yet.
First, Carracosta. I think the pros of his AV set are that he has the stats to make use of the item, with a high defense and low special defense, and he has the utility movepool to work with it. He lacks recovery however, which is a pretty big downside for what is effectively a tank, and the loss of stealth rock and shell smash hurts. This can benefit Carracosta however since most people expect him to set up with shell smash or rocks, making the opponent deal with him in a different way.
Carracosta w/Assault Vest
Ability: Solid Rock. With no speed investment swift swim doesn't benefit Carracosta at all, and with speed investment you detract from Carracosta's bulk and power. Sturdy is useful but Carracosta is designed to be bulky enough to take numerous hits, not succumb to one. The only attacks that should threaten Carracosta that far are grass type attacks, which tend to be predictable as coverage moves. Solid Rock really helps in Carracosta's bulk, since most people forget he has it as an ability so might try firing weak super effective coverage moves like hidden power which just bounce off Carracosta.
EVs: Unsure, but max HP max attack is what I use. Not all of Carracosta's moves on this set are utility based, Caracosta also wants to hit hard. Carracosta has a naturally quite low HP so that should be invested in to improve his overall bulk. With this EV spread Carracosta's defense is still higher than his special defense, so some EVs can be detracted from attack to even out the defenses. The nature I run is careful, though adamant might work better.
Knock Off
Body Slam
Aqua Jet
Coverage
Knock Off is Carracosta's main move, crippling enemies effectively. This might be especially useful in the tier Carracosta dwells, where eviolite users are pretty common. Carracosta is also one of the few to get Knock Off pre-pokebank, making this set easy to use in game.
Body Slam may not hit that hard without stab, but the paralysis chance is really nice. Combined with knock off Carracosta can go a long way in crippling an enemy team before an ally's sweep. The two moves have good synergy since ghost types that are immune to body slam are wrecked by a powerful knock off. Scald is a viable option over Body Slam, but even with stab doesn't hit as hard as body slam.
Carracosta is fortunate enough to have priority in Aqua Jet, which remedies his speed issues to a degree. Since Carracosta must also fill an offensive role this can become invaluable.
The final move can be any one of Carracosta's many coverage moves. A rock-type stab move is ideal, essentially coming down to Stone Edge or Rock Tomb. Rock Tomb has utility purpose, but Stone Edge hits so much harder and Carracosta has the bulk to take a hit if the attack misses. Crunch and earthquake are two other coverage moves, depending on what the team needs.
Oh, and Carracosta actually looks like he's wearing an assault vest.
Second, Ludicolo. That may seem like an odd option, but hear me out. Yes, Ludicolo's stats are already special defense oriented, but Ludicolo has the perfect movepool and an easy way to get around his low defense.
Ludicolo w/Assault Vest
Ability: Rain Dish. Passive recovery is a godsend for an assault vest user, though admittedly the rain nerf doesn't let Ludicolo benefit for too long.
EVs: Max special attack, max special defense, modest. HP should be minimized so that Ludicolo restores the most HP from Giga Drain, and special attack should be maximized for the same reason. Also, Ludicolo's moves do have the need to hit hard. Special defense is maximized so that Ludicolo can be as specially bulky as possible (which isn't very, but still) while being as powerful as possible. Defense is not maximised for the sheer reason that scald deters any physical attacker from coming in. Ludicolo should switch into a special attacker and proceed to wall it, while recovering HP and hitting hard with its rain boosted scald or other coverage moves. People are used to seeing Ludicolo on the offensive, and don't expect it to be particularly bulky on the special side. Even if they figure out that the Ludicolo set has unusual special bulk, they won't want to switch into a physical attacker at risk of a crippling burn.
Ludicolo definitely needs to watch out for Psyshock though.
Scald
Giga Drain
Ice beam/Icy Wind
Focus Blast
Scald is the main stab move, hitting hard under rain and providing the essential threat of a burn that keeps Ludicolo against a special attacker.
Giga Drain, combined with Rain Dish, makes Ludicolo consistently healthy. It helps that Ludicolo has superb typing with few exploitable weaknesses, making him tricky to take down.
Ice Beam hits those pesky dragon types hard, as well as opposing grass types (who cannot spore or leech seed Ludicolo). Icy Wind is viable so Ludicolo can outspeed certain threats and provide general utility for the team, but there's a pretty noticeable power difference.
Focus Blast is basically for coverage. His other moves already hit most things for neutral damage, but Focus Blast at least ensures more damage against certain steel, dark or ice types. There aren't a lot of other options for special coverage moves.
So basically, the idea of the set is to target and take down special attackers. I haven't actually tested it so it might be atrocious.
A couple of other suggestions, though ones I know are flawed, are:
Exeggutor: Higher defense than special defense, and recovery in Giga Drain. Could be pretty solid but Exeguttor needs Hidden Power so as to not be walled by steel types, and he lacks utility moves.
Hitmonchan: The base stats are already special defense oriented, but Hitmonchan has a great moveset for the item with drain punch, priority, fake out, rapid spin and PuP.
Throh: Though it misses the recovery of the resttalk set, Throh appreciates the extra moveslot to run Knock off, Body Slam and Circle Throw, making Throh potentially an excellent choice for crippling an entire team.
Hariyama: No recovery, but a high HP and low defenses, and a movepool with Knock off, priority and Force Palm could make Hariyama pretty threatening.