R. v. Pelletier, 2012 ONCA 566 deals with the law regarding unsavory witnesses - it has a helpful summary of the governing principles:
 First, as a matter of general principle, the evidence of a single witness is sufficient to support a conviction of any offence other than treason, perjury or procuring a feigned marriage: R. v. Khela, 2009 SCC 4,  1 S.C.R. 104, at para. 2.
 Second, where proof of the guilt of an accused rests exclusively or substantially on the testimony of a single witness of doubtful credit or veracity, the danger of a wrongful conviction is especially acute: Khela, at para. 2. Thus, in jury cases, we try to ensure that the jury understands when and why it is unsafe to rest a conviction on the unsupported evidence of witnesses to whom descriptors like "unsavoury", "untrustworthy", "unreliable", or "tainted" apply: Khela, at para. 3. Such an instruction, a Vetrovec caution, is not mandatory in judge alone trials: R. v. Snyder, 2011 ONCA 445 (2011), 273 C.C.C. (3d) 211, at paras. 24-25.
 Third, where a Vetrovec warning is provided, in appropriate cases the trial judge should also draw the attention of jurors to evidence capable of confirming or supporting the material parts of the otherwise untrustworthy evidence: Khela, at para. 11.
 Fourth, to be confirmatory of the testimony of an unsavoury witness, evidence must come from another source and tend to show that the unsavoury witness is telling the truth about the guilt of the accused: Khela, at para. 37; R. v. Kehler, 2004 SCC 11,  1 S.C.R 328, at para. 15. Confirmatory evidence must be independent of the tainted witness, but need not implicate the accused: Khela, at paras. 39-41; Kehler, at para. 16. Where the only issue in dispute is whether an accused committed the offence, the trier of fact must be comforted that the tainted witness is telling the truth in that regard before convicting on the strength of the tainted witness' testimony: Khela, at para. 43; Kehler, at para. 20.
 Fifth, after considering the totality of the evidence, a trier of fact is entitled to believe the evidence of a disreputable witness, even on disputed facts that are not otherwise confirmed, if the trier is satisfied that the witness, despite his or her frailties or shortcomings, is truthful: Kehler, at para. 22.
 Finally, at least in the absence of evidence of collusion or collaboration, the evidence of one unsavoury witness can confirm the testimony of another: R. v. Roks, 2011 ONCA 526, (2011), 274 C.C.C. (3d) 1, at para. 67.