What is sufficient corroboration to convict a defendant of a murder?

In People v. Vu (2006) 143 Cal.App.4th 1009 (Vu), the defendant was convicted of a gang-related murder. (Id. at pp. 1012-1013.) The court found sufficient corroboration because nonaccomplice evidence "connected [the defendant] to the crime by establishing motive and opportunity and by discrediting [his] alibi." (Id. at p. 1022.) His motive was shown by evidence that 18 months before the crime, the defendant and his "closest friend" were stabbed by rival gang members. His friend was killed. (Ibid.)