What constitutes a vouching in a witness case?

the form of vouching; how much the vouching implies that the prosecutor has extra-record knowledge of or the capacity to monitor the witness's truthfulness; any inference that the court is monitoring the witness's veracity; the degree of personal opinion asserted; the timing of the vouching; the extent to which the witness's credibility was attacked; the specificity and timing of a curative instruction; the importance of the witness's testimony and the vouching to the case overall.

United States v. Necoechea, 986 F.2d 1273, 1278 (9th Cir.1993).