Vietnam-era veterans, who for years suffered a disproportionate level of unemployment, have a jobless rate lower than that for people their age who were never in the military, according to a Labor Department study.

The average monthly unemployment rate last year was 5.1 percent for veterans aged 20 to 34, the study found.

For non-veterans in the same age group, the average monthly unemployment rate was 6.2 percent.

Moreover, the study shows that based on personal incomes (from all sources including welfare and veterans' benefits), veterans in 1977 had higher annual income in all age categories from 20 to 39 than did their non-veteran counterparts.

Veterans age 20 to 24, for example had median personal income of $6,750, while for non-veterans the figure was $5,860. In the age 35 to 39 group the veterans' median income was $16,680 compared with $15,290 for non-veterans.

The figures also show that while black Vietnam-era veterans have a much higher 1978 umemployment rate (11 percent) than the vets' group as a whole, the unemployment rate for black non-veterans in the same 20-to-34 age group is even high, at 13.1 percent.

The study, written by John F. Stinson Jr. of the Bureau of Labor Statistics and published in the Monthly Labor Review, indicates that Vietnam-era veterans have been doing better than average on the job market since 1974.

But the figures include everyone who was in the armed services between 1964 and 1975, even if they never went overseas, and thus spokesman for several veterans groups contend, tend to mask the more acute problems faced by those who actually saw combat.

In any case, a critical factor seems to be age.

If the age 20 to 34 group is broken into three categories, the figures show that in the youngest groups -- age 20 to 24 and 25 to 29 -- unemployment rates are higher for veterans. But in the age 30 34 group this reverses and the veterans do better than non-veterans.

Because most Vietnam veterans are now over 30, the lower unemployment rate for the over-30 bracket pulls down the overall average for the entire 20 to 34 category.

What this seems to suggest, according to Stinson and others, is that while Vietnam-era veterans did have greater problems finding jobs in the years immediately after discharge they eventually caught up with the non-veterans and surpassed them as they reached their 30s.

Stinson has no certain explanation for the veterans' rapid improvement, but he suggested in an interview that they may have made use of the GI Bill to pick up extra schooling in the early years, which might have begun to pay off in later years.

Stinson also pointed to the veterans preference in government hiring, giving veterans an edge in the competition for such jobs. He notes that a substantial number of veterans, particularly blacks, hold government jobs.

He also notes that some observers point out that those who go into the military must pass a physical examination and are thus likely to be healthier than those who do not, even when combat injuries are taken into account.

Stinson, and studies under a VA contract by the Center for Policy Research in New York, report that Vietnam-era veterans are more concentrated in blue-collar jobs and hold proportionately fewer managerial positions, which makes it difficult to explain why personal income in all age groups is higher. The center said it may be because veterans seem to hold more highly paid skilled craft jobs.

Steve Champlin, special assistant to the president of the Vietnam Veterans of America, a group that has called for more government job aid to Vietnam veterans, said the Labor Department figures fail to reflect crucial differences that many believe exist between those who served in Vietnam and those who served in the armed forces elsewhere.

Champlin said, "The problem is disproportionately concentrated in Vietnam theater veterans." He believes that if the VA ever agreed to make a study of unemployment rates for those who actually served in Vietnam combat, it would find the unemployment rates for them much higher than for the veterans' group as a whole.

He said an estimated 9 million Americans served in the armed forces during the Vietnam period, but only about 2.8 million actually went to Vietnam and probably no more than 1.6 million saw combat.

Dean Phillips, who saw Vietnam action with the 101st Airborne Division, then came back and sued the government in an employment case, said, "Those who were in combat face a more difficult transition to civilian life. Living with danger produces tremendous stress. When you go through a long time where people are trying to kill you it has a tremendous effect."

Champlin said that veteran's experts widely believe that combat veterans would have far higher unemployment rates in virtually all categories than would those who served in basically noncombat jobs in Vietnam, and that both these groups would have far higher rates than people who served in the United States, Europe and other places during the Vietnam period.

However, the BLS figures are based on all veterans of the Vietnam era regardless of where they served. Champlin's group would like the VA to gather statistics about each group separately, he said, but so far the request has been refused.

Champlin said studies of various types suggest that combat veterans and Vietnam-theater veterans have more psychological problems and more often fail to complete their schooling. So it is logical to assume, he said, that their job problems are greater.

The Vietnam Veterans of America is strongly backing a bill by Sen. John Heinz (R-Pa.) that, among other things, would permit a GI who served in Vietnam to be eligible for GI Bill education benefits (now $311 a month) for up to 20 years after discharge, instead of 10 as the law now allows.

It also would provide for special subsidies for employers who hire Vietnam-era veterans at salaries under $13,000, to help pay for job training and special pay allowances.

However, these provisions probably will get nowhere, because political pressure for their enactment probably will lesson as the Vietnam group ages.