Union employes of Woodward & Lothrop department stores voted overwhelmingly yesterday to approve a new three-year contract providing substantial wage increases and improved health benefits for about 6,000 workers, a union spokesman said.

About 750 of the approximately 800 members of Local 400 of the United Food and Commercial Workers who gathered at the Washington Hilton voted to accept the agreement, the union reported. Two earlier ratification votes had been postponed by snow.

The new contract, which is retroactive to Feb. 1, provides salary increases of 5 percent a year for each of the three years for most of the workers. In addition, it calls for increases in the percentages of sales that are paid out in commissions to sales personnel who are on a commission basis, such as those in the furniture and men's clothing departments.

The contract covers virtually all employes in 17 Woodies stores and three warehouses, except for clerical and management personnel. The benefits in the new contract vary according to the classification of worker, the union said, but all covered employes will receive more vacation time and increased health care benefits. In addition, Woodies agreed to post job vacancies for the benefit of part-time workers seeking full time employment.

Local 400, which says it is the largest union local in the area, has about 25,000 members. Under its contract with Woodies, all new employes are required to join the union, except those who work in Virginia. Virginia'a right to work law prohibits union membership requirements.