In the early scenes, they were gloves that belonged to Vander, a powerful freedom-fighter and essentially Vi's father figure. In the show, hextech didn't exist in their first encounter, so the gloves were essentially really heavy knuckledusters. Later in the show, after hextech is introduced, Vi scores a hextech gauntlet which turns her into the super fighter we know in the games.

Having done martial and weapon sports myself, I can tell you it doesn't take much with weapons to even the playing field against a stronger opponent. I've seen a normal-sized man KO armored fighters with a 1-foot club in ACL battles (armored combat league).

From early in the show, Vi is shown to be a seasoned fighter. A pair of heavy knuckledusters can definitely even the odds of a seasoned fighter against a larger and doped up opponent.

I think it was Jayce in his lab with all the Hextech stuff he'd created and someone(I think it was Vi, but might have been someone else) picks up the guantlets and he remarks about how he created those to help with mining. The whole thing is very clearly framed as "look at this sweet Hextech item we built with our magical Hextech".