Take a look at that definition; yeah, it's normally for NBA teams, but the size of the team is the same, so the definition should be similar.
Three all star players.
I would suggest you probably need at least the best player in two different roles, as well as having great players in others, in League of Legends. G2 last year had Rekkles, Caps, Wunder, Jankos and Mikyx - all who had an argument for best in their roles. That is a superteam.
Now, think about the best player in each role this year, and count how many of them are Vitality players. Here's a hint - it's at maximum one.
The real superteam
FNATIC, on the other hand, are the European superteam.
The best botlane in Europe - though one can argue individually Kaiser is better than Hyli - the best midlaner, possibly the best European toplaner of all time and a solid enough jungler. Compare that to Vitality's midlaner who looked probably top 3 in LCS, Carzzy/Labrov which is like... above average, Alphari who is a resource-heavy carry top laner who forces you to play around him and Selfmade, who is genuinely great.
In what lane do Vitality outclass FNATIC?
In what lane does anyone really outclass FNATIC?
They're Elyoya and Kaiser short of probably the best five players in the league, and should by all accounts smash the league up this year. Vitality would likely be happy with top three. It is NOT a superteam, it is merely a mishmash hodgepodge of decent players with some baggage.
Perkz has NA baggage, Alphari has 'doesn't win trophies' baggage, Carzzy apparently isn't rated by people any more (I have no idea why, he's pretty good), Labrov is yet to really prove he's at this level and Selfmade has the selfish tag. There's a reason all of these players were available.
The player Vitality *think* they have signed
To some extent, that is also true of FNATIC's team. And yet, Humanoid was the best Western midlaner by some distance last year, and the only real question mark is around Wunder. The only conceivable way you can argue this FNATIC roster isn't the best one in the LEC is if you really think he's a busted flush.
Please, stop calling Vitality's roster a superteam. It isn't, by any definition, and ignores the fact that we have a superteam right now.
Now, whether or not FNATIC will succeed is a completely different matter...