The California Appeals Court, Second Appellate Division, has determined that it will review the denial of a motion to vacate under section 1473.7 with a de novo standard

"De novo review is the appropriate standard for a mixed question of fact and law that implicates a defendant’s constitutional right. A defendant’s claim that he or she was deprived of the constitutional right to effective assistance of counsel presents a mixed question of fact and law, and we accordingly review such question independently. We accord deference to the trial court’s factual determinations if supported by substantial evidence in the record, but exercise our independent judgment in deciding whether the facts demonstrate trial counsel’s deficient performance and resulting prejudice to the defendant. We apply this standard in reviewing the trial court’s order denying Ogunmowo’s motion to vacate his conviction under section 1473.7, in which he argued his conviction was legally invalid because his trial counsel rendered ineffective assistance by incorrectly advising him about the immigration consequences of his guilty plea, and he was prejudiced as a result."

The Court further found that Ogunmowo's counsel (Kaplan) had performed deficiently under prevailing norms in 1989.  "Affirmatively misadvising a client that he will not face immigration consequences as a result of a guilty plea in a drug trafficking case—when the law states otherwise—is objectively deficient performance under prevailing professional norms." 

With respect to prejudice, the Court noted that "contemporaneous evidence—Kaplan’s account of discussions that occurred at the time of the guilty plea—supports Ogunmowo’s assertion he would have rejected the plea deal if his attorney had not misadvised him about the immigration consequences of a conviction. His immigration status was such an important factor to him that he affirmatively sought his attorney’s counsel about immigration consequences before entering his guilty plea. The fact that the court advised Ogunmowo that immigration consequences arising from the guilty plea were possible does not preclude Ogunmowo from establishing that counsel’s incorrect advice prejudiced him.  The court’s warning, given just before the plea is taken, does not afford the same time for 'mature reflection’ as a private discussion with a defendant’s own counsel that incorporates the particular circumstances of the defendant’s case."

The full text of People v. Ogunmowo can be found here: