This is why it's so broken. Meganium is basically our only hope and he isn't doing shit to Vileplume. Besides, HP Fire/whatever means no SDMeganium...
Probably a long, long shot but would Tropius be even remotely viable? Grass/Flying gives the STAB we need and he's fast enough to outpace the new trio of Vileplume/Regirock/Slowking.
Not if SR is up on the Tropius side or it gets hit by Regirock. I'm not too sure. That's also double Ice weakness, and I'm almost sure that Slowkings will carry at least Ice Beam or Flamethrower to fend off Grass if Vileplume goes down. It's a decent suggestion though. Maybe a quick Sunny abuser with HP Fire then to be a special version of Meganium. Get a speed boost, get rid of Regirock's sandstorm (Which I'm going to assume will always be in effect if someone's going to use Regirock, no questions asked.), and instant Solarbeams will still punish everything that comes in, resisted or not. Slap Leftovers and Clorophyll on it.