I think where the Ancients were done in is (possibly) they were so large they tried to use their insane numbers / existing tech to beat the Shivans. Then, after being worn down, the Lucy comes in and cleans their clock, and they only realize how to defeat that after it's too late (they don't have the advanced missile / bomb tech to take down the Lucy, if they even had any ships that they could scrape together to mount an offensive).
Terrans / Vasudans, however, immediately started advancing their tech and cloning Shivan tech. Thus, when they found the weakness in the Lucy, they could defeat it. This brings up the point of my post: Terrans / Vasudans probably could defeat another Lucy in short order. Track it through subspace, clobber it there. The Colossus could probably have one-shotted a Lucy in subspace, who knows. Even with the tech available shortly after the start of the Shivan incursion, if we had know to attack the Lucy in subspace, it would have slowly been worn down and destroyed after a few systems.
Come to think of it, the tech buildup for the Terran-Vasudan War probably saved both races...