Yes, VGC and OU are nothing alike. The primary reason for this is the shortness of games. In VGC a long game is 10+ turns, this gives preference for immediacy which singles formats lack. Your team needs immediate power, speed, and support. This causes many Pokemon to fill multiple roles, the Rotom formes as both support Pokemon and attackers, Quick Guard Terrakion, et cetera. This immediacy also limits the viability of Pokemon reliant on set up moves. Stall isn't viable in VGC so a predominately defense Pokemon is going to have a harder time forming a niche, however, when they do it is for their ability to support your own team and disrupt your opponent's team. Amoonguss is popular, outside of Spore, in VGC for Rage Powder a disruptive move, and in OU it gets usage for Regenerator. In terms of gaps between viability ranks, there's not really a difference. The criteria for the viability rank maybe a little different, but the range in viability suggested should be about the same.
Outside of a regional level gimmick oriented teams won't succeed in VGC. Pachirisu was on the world champions team, and could be argued to be a gimmicky Pokemon, however the team itself wasn't centered around the gimmick. Instead the team used the gimmick for a surprise support option, these kind of lesser used Pokemon being used to a high level of success isn't really uncommon in either VGC or singles provided the team and player is good.
So OU is built by players to be a competitive format,
whatever that means. Being balanced meaning all play styles are equally viable. If this was entirely successful and OU was fully balanced and competitive, then yes it would be the superior format. However, it is important to look past the ideology and at the reality. OU isn't achieving these goals and probably never will. Instead OU is an ever evolving rule set moving from one ban to the next to improve the tier. This is a process which works to an extent, the clearly unhealthy elements of the metagame are removed so the tier becomes less centralized around a single Pokemon. Thus more play styles become viable. But OU isn't balanced. VGC on the other hand is just a rule set, nothing more. VGC isn't built around an ideology, you play it because the rules are official. This means the metagame can become centralized around broken Pokemon **Mega Kangaskhan**, however, this doesn't hinder the enjoyment which can be derived from VGC. Nor does it limit the skill level of players. Be careful by what you mean by superior.
Bringing six but choosing four, increases the value of team preview and thereby your overall match-up dependency. Certain Pokemon, the teams mega evolution, will be weighted most highly. But when bringing four, the four Pokemon which you bring should be able to beat all six of your opponents Pokemon in some capacity. You shouldn't bring the four which will beat the four that you think your opponent will bring.
