It's a major plot point in both the first and second Iron Man movies that the major limiting factor in Iron Man's effectiveness, and the one thing that keeps Iron Man suits out of the reach of competitors, is power. Tony's arc reactor technology gives him the energy reserves required to run the suit and weapons that traditional power sources aren't capable of.
In Iron Man 2 a major plot point is Tony's improvement on the arc reactor technology using a new element as a fuel source. During the battle-bot fight scene Romanoff even comments on how Tony's new design is showing significantly increase power generation. This should significantly enhance most of Iron Man's capabilities, which does help explain his triumph over a literal army of bots.
Vanko's claim to fame in the movie is that he was able to replicate Tony's original palladium based arc reactor design. As such we can assume that both the battle bots and his suit design had approximately the same amount of energy available as Tony's original suit. This is further supported by Vanko taunting Tony on their phone call by noting how he took Tony's advice to increase power output.
Vanko's suit was significantly larger than Tony's, with an implied higher mass. This means that most any functionality, from moving to punching or flying, should require significantly more energy. I'm not sure, but I thought this was even noted in one of the movies for the reason Tony's suit was as small as it was. In any case, given that Vanko's suit had significantly less power available while also requiring far more power for even the most basic actions, why was he able to match Tony in firepower, dexterity, flight speed, and all other dynamics during their battle, requiring two of Tony's suits to take down?