I feel like we need to distinguish the intention here.
If we assume a character is top tier we can mean a number of things.
To me, top tier in a team game is measured by the characters impact on the match as a whole. How much power he brings to a match and what win conditions he enables. Additionally, the flexibility of the character and the options he brings should have strong weight as well.
So, by that metric, Vegeta is top tier. Period.
To some people though, they may be describing the experience of playing the character directly. Instead of looking at the larger impact on the game they focus in on the singular experience of playing the character. Ie, when I have this character out and am using him, how powerful is he, what are his options (for and against) and what are his win conditions? We are *just* looking at his impact when he is OUT and deciding based on that.
So, by that metric, Vegeta is (imo) high tier. He's not Cell. He's not AG. but he does have very very strong options, they just arent overly flashy or crazy. He isnt about flashy resets or 2min blockstrings or anything. He just plays a powerful, fundamental game and does good damage when he touches you. He gets good mileage from meter and he can contest in neutral vs most characters. His tools are simple to implement and understand and his win conditions are simple, and broad. You dont need special scenarios to use him correctly or get the most out of him. He doesn't need to heavily rely on assists to work or do damage and he has strong defense with his ground and air invincible DPs. He isnt top tier in THIS metric though, because other characters like Cell and AG and such, simply outstrip him.
I dont personally think that looking at a character in a vacuum is the right way to go about it though. A character should be defined by the impact they have on the game as a whole. It's just how things actually work. Yeah, Vegeta alone may not be as powerful as Cell or AG or 16 or whatever, but BEFORE (assuming you're running him anchor and you probably should be) that happens, he makes your other two characters much stronger, allowing far more options, win conditions, pressure, everything.
I think the issue we have here is just intentions. Osari is just saying that he, when considered alone, is not a top tier character with top tier tools and such. He's mid-high (imo he's High, but its certainly arguable) when you're PLAYING him, ie, he doesn't have his own assist to do Vegeta assist things with. And this isnt wrong and its not exactly the wrong way to look at it - provided I think you allow that ALL THINGS CONSIDERED, he's a top tier character.
If Osari is saying he thinks Vegeta, all things considered, ie, we make a tier-list encompassing the entire game's cast, and the list is inclusive*, should be ranked mid-high.. I do think that's incorrect. Inclusively speaking, he is a top tier character. If we make a list geared towards individual performance and strength, then I see what he's saying completely. I just think a list like that is more for fun/talking points/discussion generation, than having any statistical value.. since the game doesn't exist in that vacuum and under those circumstances.
(ie, not geared towards something specific, like tier list when fighting alone, or tier list of points, or blah blah)
My personal tier list I'll leave at the bottom of this post. Characters are not ordered within tiers. Please remember that no matter where I put a character, everyone can do damage, everyone can win. No one is outright bad. I dont order it into S A B, etc either.
Always Very Strong. Characters that are just a cut above. They bring extremely powerful options and can heavily influence the game regardless of what's going on. If you're trying to win a tournament, this is where you start,
Black - yep.
16 - of course.
Cell - "Helloooo"
AG - umm..
Kid Buu - jhksgdb8732y48uhfidufds7.
SSVegeta -the two s's are from the word aSSist.
Always Quite Strong. Characters that bring powerful options to the table, with those options being universally useful and not requiring specific setups or situations. Ie, no matter what's happening in the game, these character pose a strong threat.
21 - steal the right moves and shes top. without, shes just plain good.
Trunks - hes good. pressure. he spins.
Hit - *joke about him saying THIS IS THE STANCE*
Blue Goku* - maybe better but I dunno. Just my personal understanding of him. He does a lot. hits hard.
Bardock* - I know its early, but everything shows him shaping up to be a beast. Rush and pressure.
18 - Being left behind by a lot of people but still super strong.
SSGoku - He's just plain good. A threat.
Good, but true strengths are often a little situational, or occupy a narrow spectrum, ie, they do ONE thing very well.
Piccolo - needs those knockdowns and such, but has a decent base kit to get them going.
Tien - He's good, but making proper use out of his ability to delete another character can be tricky. His overall kit is good though.
Frieza - Might be moved up a tier. Has lots of stuff. Just never see him live up to his 'on paper' potential.
Good, but very situationally so. These characters can do work but often require specific conditions to shine. Or true strengths occupy a very narrow spectrum.
TG - When things go right, hes TERRIFYING. When they dont, he's a liability. Needs two assists, momentum and lots to go well in order to make him shine.
Majin Buu - Same as TG
Ginyu - Same as TG
Beerus - Just feels lackluster. Lots of work for mediocre return, needs setups and such to do work, but doesnt easily get them.
Krillin - Super fun rushdown and lots of neat shit. His flips and shit are dope, but lackluster over-all and requires a ton of work.
Nappa - Awesome character, but needs a pair of assists and quite a lot to go well to really shine.
I dont know. These characters I am just unsure about. I havent seen them used enough to make any kind of call, but ill list where I think they might fall.
Broly - No idea. he has some CRAZY shit. I think he'll be under top tier, but still strong.
Yamcha - Honestly? Speak to one person, Yamcha is SSSSSSSSSSSSSS+. Speak to someone else and hes D-. Watch him play and scratch your head. No idea. He has some shit, but a lot of bullshit or can be mashed out. I rarely see him used anymore. People get hyped, then honeymoon phase ends and they drop him.
Gotenks - If the game as a sleeper, I think he could qualify. I think he;s middle of the pack possibly but I dunno.
BlueVegeta - No damn clue.