What is the test for a motion to vacate a sentence imposed in violation of the Constitution?

Section 2255 provides that a prisoner in federal custody "may move the court" to vacate a sentence that "was imposed in violation of the Constitution." 28 U.S.C. 2255(a). Broxmeyer claims that his sentence was imposed in violation of his Sixth Amendment right to the effective assistance of counsel. To establish ineffective assistance of counsel under Strickland v. Washington, 466 U.S. 668, 686 (1984), the defendant must prove both "that counsel's representation fell below an objective standard of reasonableness . . . . under prevailing professional norms," id. at 688, and "that the deficient performance prejudiced the defense," i.e., "that counsel's errors were so serious as to deprive the defendant of a fair trial," id. at 687.