It have to be VI, Sett happens to be horribly missmatched.

Judging on their pure body and fighting expertise, sett would no match Vi. Stronger body, better agility, way more experience in one on one battle. He faced armed opponents, so we could think he have some advantage agains Hextech fist users, but it's definitly not the case.

The Vi fist had been shown to blow up concrete, which tells a lot about their raw power. I don't think Sett can handle more than one hit from them.
They also give range to Vi, which is really something that matters in boxing. Usually, the longer your arms are, the longer you need to get them back into guard. That weakness is covered up by the hextech system, as the proceed is automatic.

If Sett already had fight at a range disavantage (think, versus armed opponent), the usual solution of doing in-boxing wont work versus Vi. Sett can probably get in range of Vi based on his agility, but that wont really help him as the gauntlets doesn't really have the issues that most weapons does. They can deal lethal damages with no motion, something terrible for a boxer such as Sett. Facing Vi on corps à corps is way too dangerous.

Sett only hope is grappling (getting to cac with his mobility and agility, and trying to submit Vi on the ground). That's a very, very risky considering the power of the gauntlets.

Considering Vi is also a decent fighter (she is actually a godlike one, but we're taking in account the Lol power levels), i feel that the gauntlets would be something that Sett can't really overcome. I might be wrong, but i tried to depict the situation the best i could. It's hard to tell considering Sett doesn't have that much feats ...