I get the impression, that the Vord's goal is total eradication of all life that's not them.

So, if they achieved it, what would they eat?

Those in Wax Forest, back in "Furies of Calderon", seemed to be surviving on animals that fell or wandered in (like that crow whom Tavi tried to free from the Croach), and the occasional Marat, who failed to stay sneaky enough and was caught (Kitai said something about it, as far as I remember).

But if they killed everything there is to kill, and covered all the ground with Croach, it seems there'd be nothing left for them to eat.

Well, there were trees in Wax Forest (if they were still alive), but I got the impression Vord are carnivores, or at least getting whatever food they keep in and transport through the Croach from animals & people, not plants.

In fact, what did they eat in the areas they conquered and covered in Croach (like the majority of Alera at the time of Third Calderon) ?

I'm reading "Grave Peril", currently at the party at Bianca's, where Lea claimed Harry broke the deal he made with her three times: one years ago, one a few days back, and one just a day ago or so when saving Charity Carpenter at the graveyard.

I remember two of those (years ago, when fighting Justin or his demon, and the one when saving Charity), but when did the third (technically second?) one happen? The only meeting they had in the meantime, that I remember, was when going after Agatha Hagglethorn's ghost in the Nevernever (from the hospital's maternity ward), but I don't remember Harry promising anything to her back then? (unless "I will go with you... when Hell freezes over!" somehow counts?)