In what circumstances will a jury consider vouching a witness in their opening statement?

In United States v. Shaw, 829 F.2d 714, 716-18 (9th Cir.1987), cert. denied, 485 U.S. 1022, 108 S.Ct. 1577, 99 L.Ed.2d 892 (1988), the prosecutor told the jury in opening statement that the defendant's accomplice and an important government witness had agreed to testify and that "we ... have agreed that as long as he is truthful we will present his truthful cooperation to the local prosecutor." Id. at 717. The court instructed the jury that the witness was the beneficiary of a plea bargain and that the jury should examine his testimony with greater caution than that of ordinary witnesses. Even though the prosecutor's words imply that the prosecution had some method of determining whether the witness's testimony was truthful, and communicated a clearer message coming at the outset of trial before credibility had been challenged, we concluded that the vouching was harmless error. Id. at 717-18.