You're confused as to the mission of the Midway.
Midway is not an all-powerful super-ship to replace the Vesuvius' best carrier honors -- rather, it is a *cost effective* heavy carrier. The Midway is designed to function as a cheaper ship which can fullfill more roles than the carrier only Vesuvii. As a carrier, Vesuvius is far superior -- and is clearly still a major player in 2681.
The Vesuvius class ships are still a major player in 2681 -- as carriers, they are far superior to the cheaper Midway class alternatives... Midway was designed to be cost effective, Vesuvius wasn't.
The display on the screen about Vesuvius is showing the position of her ion turrets -- but she has dozens of defensive turrets, as well as a number of offensive anti-matter guns. In a one on one fight, though (as SO shows us) a lone Vesuvius will not survive (she does indeed outgun Midway, though).