Is it permissible for a federal prosecutor to vouch for the credibility of a witness?

Although "prosecutors may not vouch for their witnesses' truthfulness," the Government is permitted "to respond to an argument that impugns its integrity or the integrity of its case." United States v. Carr, 424 F.3d 213, 227 (2d Cir. 2005). "In a particular context . . . what might superficially appear to be improper vouching for witness credibility may turn on closer examination to be permissible reference to the evidence in the case." United States v. Perez, 144 F.3d 204, 210 (2d Cir. 1998).