We never get the story directly but you can put it together from a couple of different places!
In Fleet Action, the Concordia is badly damaged in the Battle of Earth. She takes three torpedo hits by the end of the story "and was crippled, barely able to make twenty percent speed" when the book ends. Then the first diary entry in the Wing Commander Armada manual, Voices of War, tells us she didn't really recover and was instead assigned to patrol duty. "The Concordia was growing rather rickety after years of abuse, and she’d been placed on indefinite patrol status, Code 4. What this translated into was running routine 'safe' sector patrols. So I did just that, flying sortie after sortie in clear, open space. For eight months straight."
Then Wing Commander III novel tells us that the she was lost in a rearguard action. We find out that Blair took over as Concordia's Wing Commander after Angel left for Covert Ops but that he was wounded at the Battle of Earth and spent months recovering... so he wasn't aboard when Concordia was destroyed. (Tolwyn would've left after Earth, too, as he was promoted to command the Third Fleet and then the Weapons Development Office.) It establishes that Vespus is one of the major Confederation systems in the area but that it was well behind the lines at the time--it's the place soldiers go for R&R, they didn't think the Kilrathi could strike Vespus.
And then we have the reference to Strakha from Star*Soldier! So the Concordia, reassigned to avoid action was protecting the rear when the Kilrathi attacked with more strength than expected. When we see Kevin Tolwyn in the WC3 novel he mentions he was promoted after heroism in the "mop up after Vespus" so you can imagine the main body of the fleet coming back and forcing the Kilrathi back out after Concordia is destroyed in some desperate defense.
