I don't think Decidueye will ever run an Assault Vest set, not gonna lie. Its physical bulk is far too low for that; most viable Assault Vest users have decent physical bulk, like Tangrowth and Conkeldurr, though there are some exceptions, like Tornadus-T, which has Speed in place thereof: Decidueye lacks both. I think its best set will be a Swords Dance set because it capitalises on Decidueye's nice special bulk to give it an easy time setting up. You seem to be misunderstanding the use of the Assault Vest; it's usually for slower Pokemon with low Special bulk, or faster Pokemon that enjoy the extra special bulk. With Decidueye's base 70 Speed bracket, it is neither. Its Special bulk is already so nice, anyway. Additionally, you have absolutely no longevity as you cannot run Roost, Synthesis, or Leftovers, and are locked out of the ever-important Sucker Punch and Decidium Z. With Brave Bird, you are worn down even faster, too. Don't run Assault Vest, it is incredibly counter-productive and an on the whole useless item.
Decidueye @ Decidium Z
Ability: Overgrow (or Long Reach when it's released)
EVs: 252 Atk / 4 SpD / 252 Spe
- Swords Dance
- Spirit Shackle
- Leaf Blade
- Sucker Punch
- Swords Dance allows you to double your Attack stat, allowing you to double your physical damage output.
- Spirit Shackle is one of the strongest (and arguably the most viable) physical Ghost-type moves in the game. Unlike Shadow Claw, it has a nice Base Power, and unlike Phantom Force, the opponent cannot switch out into a better matchup easily. Not to mention that it traps the opponent in, too. All in all a fantastic move, and is definitely among one of Decidueye's strong points. With a Decidium Z attached, Decidueye can reliably nuke many threats and, to a lesser extent, physical sponges, thinking they can take the hit with ease.
- Leaf Blade is reliable Grass-type STAB, with a fantastic high Critical Hit rate alongside a great Base Power of 95.
- Sucker Punch allows you to pick off faster, more frail threats, thinking they can revenge kill Decidueye. Even with 252 Speed and a Jolly nature, Decidueye only hits 262 Speed; this is very mediocre, and makes Decidueye lose to most faster attackers, giving it a weak matchup against most Hyper Offensive teams.
Conclusion: Decidueye is a great sweeper by virtue of its arsenal of the best physical Ghost-type move in the game, Spirit Shackle, which gains STAB with its Ghost typing. This, combined with the Decidium Z Crystal, gives Decidueye Sinister Arrow Raid, a 195 Base Power Ghost-type move that OHKOs pretty much everything in its path after a Swords Dance boost. Its access to Sucker Punch, the second strongest priority move in the game, means revenge killing it is difficult for most non-Dark resistant faster attackers. Defensively, it has a fantastic support movepool at its disposal, but finds it difficult to make use of it, what with its mediocre physical bulk, though its special bulk should not be overlooked; base 100 is great, especially for a largely-offensively inclined Pokemon. The Assault Vest set should not be considered by any means, as it locks Decidueye out of Roost, Defog, Haze, Toxic, and the like, meaning Decidueye has horrible longevity for a defensive Pokemon; all it does is give it nicer special bulk, which it doesn't need. It should only utilise an offensive set due to these traits.
Other options: Decidueye can reliable run a Choice Scarf set, hitting 393 Speed, which makes it fall just short of the 130 Speed mark, which hits 394. This is nice and all, what with Decidueye's great Attack, but it is still underpowered by comparison to other sets, makes Sucker Punch useless, and locks it out of Sinister Arrow Raid. Still something to consider if your team needs something reasonably fast, however.