In addition to the cost savings, I suspect what they're doing by waiting until he's released they can make sure that the subsequent trial (and presumably conviction) won't be concurrent.
Sorry, you are thinking like a normal person, not the way Surry county prosecutor Poindexter thinks. He hasn't wanted to prosecute Vick since day 1. He's recently fired an assistant who went on the record that Poindexter did not view the Vick thing as a big deal, which is why the assistant chose to get the fed involved in the investigation, prompting the fed to take action. When the fed got Vick to admit to everything, Poindexter had every opportunity to get Vick indicted for crimes he could prosecute under Georgia
Virginia law. For example, he had a chance to get a grand jury to indict for the killing of animals, which had not been prosecuted under federal law. Instead, despite Vicks confessed involvement in the killing of unfit dogs, and the discovery of dog corpses, he got an indictment for the same animal cruelty charges which had been brought by the fed, and which Vick copped to. Poindexter has already acknowledged publically the result is these Georgia
Virginia charges will be thrown out eventually under a Georgia
Virginia law which prohibits state prosecution for a crime already tried under federal law. Avoiding concurrent sentencing is the last thing on Poindexter's mind. Once Vick is out of his federal sentence, he has absolutely nothing to worry about from Surry county.