So, you're saying Vileplume deals with the likes of Meganium worse than Meganium, who has no Sludge Bomb at all, would?
You're saying Vileplume handles Fighting-types worse, despite resisting Fighting-type attacks, while Meganium does not?
You're saying Vileplume's Base 100 Sp.Attack dosen't hit as hard off the bat as Meganium's Base 82 Attack or 83 Sp.Attack does to counter things?
Sure, Meganium has more bulk, but Vileplume's typing is better suited to defense, and it can deal more damage to the Grass-types that threaten Regirock and Slowking as well. Not to mention x4 Resist to Grass.
Oh, and, this is how things would go:
Meganium switches in. Takes 12% from SR. Maybe Spikes damage too. Also takes damage from whatever it switches in on. Or, god forbid, Slowking T-Waves it, making Vileplume outspeed, or it takes a Toxic. This is a Stall-based team it's facing, after all, so some form of residual is a given.
Let's not forget Sand Damage is possible, and likly, with RegiKingPlume too. Meganium is already between 94% and dead, probobly around 70% -> 80%, and maybe statused.
Meganium SD's. Vileplume gets in.
4 Sp.Attack, Neutral Nature Sludge Bomb v 4/0 Meganium:
64.2% - 76.2%
+2 Return v 252/0 Vileplume [My personal set], with Life Orb and Adamant Nature:
75.1% - 88.7%
No OHKO there, meanwhile, the above Meganium has probobly just fainted due to LO recoil, if it had taken any attack from what it switched in on, or Sand/Spikes were up. If it got T-Waved, it dies due to Sludge Bomb next turn.
Same Meganium v 252/252 Bold Vileplume:
52.5% - 61.9% Actually, this dosen't change the HKO number, but it would if Meganium lacked Life Orb, or was Jolly.
Speaking of:
36.7% - 43.5% ~ Jolly Non-LO Meganium deals at +2
Meganium neither counters RegiKingPlume, nor is superior in assistance to RegiKing.