In a throwback to DPP UU Venusaur, Vileplume can pull off a remarkably similar special attacking set to what was arguably the most prominent threat in that metagame; by utilizing the exact same STAB coverage and Special Attack stat that made Venusaur so fearsome, Vileplume is a very potent threat in the RU metagame. First and foremost, Vileplume has Sleep Powder to incapacitate the few Pokemon that can stand up to her powerful STAB coverage. Furthermore, there is nothing that compares to Vileplume's combination of bulk and power in RU: Amoonguss is too slow and weak, Roselia is weaker due to her need to use Eviolite, and Victreebel is incredibly frail. The only thing that holds Vileplume back is her poor Speed, but she can still outrun many common RU walls and deal massive damage off her good Special Attack. Overall, this Vileplume makes a fearsome tank. Like DPP UU Venusaur, she can switch into many prominent threats such as Sceptile (beware of Acrobatics), Tangrowth, Rotom-C, and Slowking and dish out incredibly powerful attacks to continually wear down the opposing team.
Sludge Bomb and Giga Drain are Vileplume's quintessential offensive attacks, and they have remarkable coverage together. Sludge Bomb is Vileplume's main attack due to its significantly higher Base Power and excellent neutral coverage, while Giga Drain dents many common walls, such as Slowking and Poliwrath, and restores a significant amount of Vileplume's HP. With good prediction, the combination of these two moves will break down many common RU cores, including the Tangrowth and Slowking core. Because this requires an incredible amount of prediction to pull off successfully, Sleep Powder is a fantastic move, as it can simply be thrown out to cripple whatever switches in, should you feel unsure of your prediction skills. Sleep Powder also allows Vileplume to circumvent some of her counters by putting them to sleep and then bashing at them with her incredibly lethal STAB attacks. To round out this set, Moonlight gives Vileplume a very reliable form of recovery due to RU's lack of weather, and it helps mitigate Life Orb recoil, as well as damage from entry hazards and attacks, allowing Vileplume to stay alive to check troublesome threats and smack opponents with her powerful attacks.
The given EVs allow Vileplume to outspeed uninvested base 70 Speed Pokemon such as Poliwrath, which allows her to quickly nail them with a powerful attack or Sleep Powder. Maximum Special Attack investment is highly recommended on Vileplume to make her as strong as she can possibly be, while any remaining EVs should be dumped into HP to give Vileplume some extra bulk. Maximum Speed investment with a Timid nature is another decent option, as Vileplume still hits respectably hard and becomes able to outspeed troublesome foes such as Adamant Bouffalant and 2HKO them. On the other hand, fewer Speed EVs can be used if bulk is more important: 142 Speed EVs let Vileplume outrun uninvested Lanturn, 124 Speed EVs beat out uninvested base 65 Speed Pokemon such as Sandslash, and 84 Speed EVs can be used for base 60 Speed Pokemon such as Clefable. It is not recommended to run fewer Speed EVs unless you simply want to maximize HP, as beyond this, most Pokemon tend to use at least a little Speed investment. Effect Spore is the recommended ability; though unreliable, the 30% chance to inflict a status condition on the opposing Pokemon can come in handy, and Vileplume is too slow to make use of Chlorophyll. However, note that Chlorophyll has no move restrictions while Effect Spore does; moves such as Leech Seed can only be used with Chlorophyll. Hidden Power Fire can also be used over Moonlight if you desperately need a way to remove Steel-types.
To show how powerful and bulky this Vileplume is, here are some damage calculations:
Sludge Bomb vs. +1 4/0 Lilligant 110.64% - 130.50%
Sludge Bomb vs. 0/0 Accelgor 92.69% - 108.97%
Sludge Bomb vs. 4/0 Scolipede 47.71% - 56.11%
Sludge Bomb vs. 0/0 Moltres 65.73% - 77.88%
Sludge Bomb vs. 0/4 Bouffalant 58.31% - 68.88%
Sludge Bomb vs. 252/0 Uxie 41.81% - 49.44%
Sludge Bomb vs. 248/116+ Slowking 35.37% - 41.98%
Giga Drain vs. 252/0 Poliwrath 83.33% - 98.44%
Giga Drain vs. 52/204+ Lanturn 65.35% - 77.23%
Giga Drain vs. 248/116+ Slowking 56.49% - 67.18%
Giga Drain vs. 0/0 Aerodactyl 61.79% - 72.76%
252 SpA Life Orb Accelgor Bug Buzz 57.24% - 67.43%
252 SpA Life Orb Lilligant Hidden Power Fire 63.16% - 75.00%
+1 252 SpA Lilligant Hidden Power Fire 73.68% - 86.84%
+1 252 SpA Life Orb Lilligant Hidden Power Fire 95.39% - 112.50%
252 Atk Life Orb Adaptability Crawdaunt Crunch 80.92% - 95.39%
252 Atk Life Orb Sawsbuck Return 68.09% - 80.26%
252 Atk Life Orb Scolipede Megahorn 75.00% - 88.16%
252 Atk+ Choice Band Aggron Ice Punch 90.13% - 106.58%
As you can see from these damage calculations, entry hazards are lethal for Vileplume, as she can just barely survive the attacks from some of RU's most powerful attackers. Furthermore, with entry hazards, Life Orb recoil, and damage from attacks as she switches in, Vileplume will not last as long as one might like. As such, Rapid Spin support is recommended to increase Vileplume's longevity and allow her to keep dishing out powerful attacks. Kabutops, Sandslash, Hitmonchan, and Cryogonal make wonderful spinners to use alongside Vileplume. The former two are able to set up Stealth Rock for Vileplume in addition to spinning away entry hazards, while the latter two are extremely reliable spinners. Entry hazards of her own are also beneficial, as demonstrated by the above damage calculations. In addition to Sandslash and Kabutops, Omastar, Ferroseed, Qwilfish, and Crustle make excellent entry hazard users, as they can each use two different types of entry hazards to assist Vileplume.
This set's main adversaries are defensive Poison- and Steel-type Pokemon, as they resist Vileplume's STAB combination and Sleep Powder can only do so much in dealing with them. Magneton makes a wonderful partner because of this, as it is immune to Poison-type attacks and can easily trap and KO troublesome Steel-types such as Ferroseed, Klinklang, and other Magneton. Alternatively, Ground-types such as Steelix, Sandslash, Golurk, and Rhydon can wield a STAB Earthquake that hits both Poison- and Steel- type Pokemon super effectively. Specially defensive Pokemon such as Munchlax, Cryogonal, Clefable, and Mandibuzz might also pose an issue if an opposing Pokemon is already asleep, so powerful physical attackers, particularly Hitmonchan, Entei, Aggron, Feraligatr, and Medicham, are recommended partners. Entei and Feraligatr also share wonderful type synergy with Vileplume, as Vileplume works well in the traditional Fire / Water / Grass core due to its members' abilities to take on each other's weaknesses extraordinarily well.