Ms. Vann was responsible for hiring Omnicom Group's BBDO Worldwide, New York and Atlanta, the company's agency of record.
A Cingular spokesman said the estimated $400 million account is not in review.
Held job since launch
Ms. Vann had held that post since the launch of the new consumer wireless brand two years ago.
The resignation was spurred by the arrival of a new president-CEO at Cingular, Stanley Sigman, who was named on Nov. 6. Formerly the chief operating officer of SBC Communications, Mr. Sigman made the CMO function report to Mark Feidler, chief operating officer.
Previously, Ms. Vann reported directly to Cingular's former president-CEO, Stephen Carter, according to a person familiar with the situation.
For the near-term, marketing will report to Mr. Feidler; a search is on for Ms. Vann's successor.