First of all, it doesn't even come close to "outclassing" Tangrowth offensively. In terms of stats, the two are equal Special-wise, at 110 each, and Tangrowth has a good 20 points on Vileplume in attack, at sitting at 100 while Vileplume has a much lower base 80. Second of all, Tangrowth has a well above-average attacking movepool, carrying Giga Drain, Sludge Bomb, Knock Off, Power Whip, EQ, Focus Blast, and even Rock Slide. Vileplume only has access to a couple of these. It does carry Dazzling Gleam, so I'll give you that, but it certainly isn't better offensively.
Secondly, Amoonguss is much better than Vileplume defensively. Vileplume doesn't carry much weight over Amoonguss in Defense and Special Defense, at 85/90 over Amoon's 70/80, however, Amoon's 114 HP is marginally larger than Vileplume's 75, making it much bulkier overall. Amoonguss also has access to Spore, which is the biggest factor. You said: however, this is definitely not true. Amoonguss can use Stun Spore, but almost always doesn't simply because Spore is a better move. On top of that, Amoonguss can carry Effect Spore, but doesn't because Regenerator is a much better ability.
I'm not really sure about the last part because it can't really break that Regen Core (outside of sun?) and 3HKOing Registeel isn't significant considering Registeel can Paralyze you and proceed to Paraflinch you to death.
