Two-thirds of the nation's workers age 25 or older worked for firms with pension plans in 1984, the Census Bureau reported yesterday.

Based on surveys taken in the fourth quarter of 1984, the bureau found that 78.6 million people had worked in a wage or salaried job in the four months preceding the interview, and 52.7 million (67.1 percent) were in firms with pension plans. About 35 million of the total were already vested for benefits, which means that they had earned an unforfeitable right to receive benefits in the future, the bureau said.

The survey dealt only with employer-sponsored pension plans, and did not consider Social Security coverage, independent retirement accounts (IRAs) or savings plans such as the tax-deferred 401(k) plans.

Pensions included those earned in private employment as well as state, local and federal government employment and military service.

Officials said the coverage proportion would have been lower if surveys had included workers under 25, many of whom work in temporary or entry-level jobs in firms where no pension coverage is provided.

The survey found that coverage was highest for men, high-paid workers, public employes, career military personnel and employes of large firms.

Thus 70 percent of men and 64 percent of women were covered.

Only 37.8 percent of workers making under $500 a month were covered, compared with 84 percent of those at $2,000 or more.

In the armed forces 100 percent were covered; in state, local and federal employment at least 93 percent at each level; in the retail trades 46 percent; in personal services 22 percent and in manufacturing 76.5 percent.

In firms with fewer than 25 workers 25 percent were covered; in firms with 1,000 or more workers, 90 percent.

The survey also found that aside from pension coverage, 16.2 million of the 78.6 million workers had IRAs. More than 8.6 million were eligible for 401(k) plans; 4.8 million participated in the plans. For both IRAs and 401(k) plans, participation was highest among the best-paid workers.

The survey, looking at people who were actually receiving pensions, found that 11.5 million people were pension recipients in 1984, averaging $568 a month in benefits. Of the 11.5 million, about 2.3 million were under 62