The National Association of Letter Carriers has overwhelmingly approved a new three-year contract while voting to oust its president, J. Joseph Vacca, union sources said yesterday.

Neither development came as a surprise in light of internal union turmoil stemming from recent contract bargaining that kept the country on the verge of an illegal postal strike for more than a month last summer.

The vote to ratify the contract further reduces any chance of a strike, but the 180,000 member union's rejection of Vacca in favor of Vincent Sombrotto, head of the NALC's New York branch, points toward more militancy among postal unions in the future.

The contract, imposed last month by a special federal mediator, gave workers higher pay and cost-of-living protections than an earlier pact that was negotiated by Vacca and other union leaders and then decisively rejected by rank-and-file union members.

The new contract calls for a pay increase of at least 21.3 per cent over the next three years if the inflation rate exceeds 6.5 per cent a year. The rejected contract envisioned a 19.5 per cent increase over three years.

The contract imposed by the special mediator, James Healy, was ment to be binding under an agreement worked out between union leaders and the Postal Service. However, Vacca and Emmet B. Andrews, president of the American Postal Workers Union, decided to submit the settlement to a rank-and-file vote anyway.

Although postal strikes are illegal, Vacca had said his union, second in size to the 280,000 member APWU, would walk out if its members rejected the contract.

The APWU is also expected to approve the new contract. Andrews, like Vacca, is facing an election challenge but has been described as being in less trouble.

The third union involved in the postal negotiations, the Mailhandlers' Division of the Laborers International, accepted the mediator-imposed contract without going to a referendum.

The new contract envisions an increase in average postal pay from $15,887 about $19,500 by 1981, with the exact figure depending on the inflation rate. One change imposed by healy was to lift the existing ceiling on cost-of-living increases. Another was to modify the prevailing "no-lay-offs" clause to guarantee lifetime jobs for current employees but make it possible to lay off workers hired in the future.