Leo M. Bernstein announced yesterday that he has resigned as vice chairman of National Savings & Trust Co. to become president and chief executive of the Diplomat National Bank.

Bernstein's purchase of controlling interest in Diplomat was approved by the bank's board of directors Friday afternoon.

The change in ownership, which must be approved by federal bank examiners, represents the first time in the five years since Diplomat was chartered that control has slipped away from South Korean interests.

Control by Koreans has caused controversy for the bank following disclosure that such control was backed secretly by the South Korean government and agents of evangelist Sun Myung Moon.

In taking over the top post at Diplomat, Bernstein said, "I hope to broaden the base of the bank's operations to serve not only the Asian-American community, which founded the bank, but also all minorities, the business community and the residents of Washington."

Bernstein won his bid to take over the bank after the first choice of Diplomat's board, a group of South Korean investors headed by that country's largest bank, Cho Heung Bank Ltd., failed to win quick certification from the U.S. Comptroller of the Currency.

Diplomat's current president, John Ricche, will be offered other duties at the bank, a spokesman for Bernstein said.

Bernstein's purchse of 500,000 shares of the bank's stock represents an 11th-hour rescue of the financially troubled institution, which has been under orders from the comptroller to either raise at least $1 million in new capital or face closing its doors.

In his statement, Bernstein, said that he plans to "devote my full time, attention and energies to making the new bank a successful and profitable institution."

Specific long-range plans for the bank are expected to be announced at a press conference Tuesday, a Bernstein spokesman said.