What is the test for reversing a conviction on grounds of improper vouching?

Reversal of a conviction on grounds of improper vouching is justified only if a prosecutor's improper remark "causes the defendant substantial prejudice by so infecting the trial with unfairness as to make the resulting conviction a denial of due process." United States v. Carr, 424 F.3d 213, 227 (2d Cir. 2005) (quoting United States v. Shareef, 190 F.3d 71, 78 (2d Cir. 1999). Where, as here, the defendant did not object at trial, and we review for plain error, we will reverse a conviction only in instances of "flagrant abuse." Id. (quoting United States v. Zichettello, 208 F.3d 72, 103 (2d Cir. 2000)).