Is it fixed? What tier?
>immunity to status
Given the context of the OP, yes, because it would be enhanced by Dragon Dance.
Makes sense too given it can learn Vine Whip. This is just another case of Gamefreak being stupid.
>Flower Veil also prevents the Pokémon with this Ability (if it is Grass-type) and its Grass-type allies from being afflicted by non-volatile status conditions or Yawn. However, it will not prevent a Pokémon from falling asleep if it is already drowsy from Yawn. This Ability has no effect on Rest, the Flame Orb, and the Toxic Orb.
Non-volatile status include Burn, Poison, Paralysis. Doesn't work with Toxic Orb and Burn Orb, though.
meganium has two nice things: first it's a grass type so it has cool moves like leech seed and aromatherapy, and second it has earthquake which is really nice and complementary as grass type.
looking at and wanting to work with this, the first big idea that comes to mind is to turn meganium into a bulky physical attacker, with the movepool and stats to do tons of annoying damage but also help the team.
unfortunately meganium misses out on a bunch of really important moves for a supporter, like stealth rock and sleep powder.
so whatever you do with it, keep in mind it's always going to run basically the same set.