Why is the formula in the quote box (the correct one) different from the one in your post (the incorrect one) without me making any edits? Because that is the source of error, an incorrect formula.
Side note: A quick way to determine what a pokemon would outspeed at +1 is base*1.5+24.25. Anything with a base under that is outsped, anything with a base over that outspeeds you. For +2 it is base*2+49.5.
I edited my post, realizing I posted the first and second generation instead of the third gen on. You must've been on the page before I made the edit, and replied afterwards. I redid the calc, and it turned out fine.
Why do people use assault vest T-Growth leech seed partly makes it the wall it is right?
If used as a wall, you're right, Leech seed should be used (and assault vest shouldn't). But the most common set for tangrowth is a bulky pivot, that can take hits from both sides and do damage with its great coverage. On that set, it has Regenerator, so Leech seed won't be missed that much.
How does huge/pure power work? like if my azumarill's base atk is 80, is it 160 now?
Azumarill has a base attack of 50, not 80. The maximum attack for a pokemon with 50 base attack is 218, so Huge/Pure power doubles the stat itself, not the base stat. For Azumarill, it comes out to 436, which is comparable to adamant/etc. pokemon with a 150 Base Stat.
Also, next time, try to post both questions in the same post :)