Final preparations were under way yesterday for the National Salute to Vietnam Veterans, a panoply of ceremonies, concerts, vigils and reunions that begins here tomorrow and culminates Saturday with the dedication of the Vietnam Veterans Memorial on the Mall.

"All plans are operational and we are charging forward," Jan C. Scruggs, the founder and president of the Vietnam Veterans Memorial Fund, said yesterday.

Workmen started constructing bleachers and a reviewing stand along Constitution Avenue NW for Saturday's 10-block-long parade, while another crew was building the podium above and to the rear of the controversial black granite memorial in which the names of the 57,939 war dead and missing are carved.

Scruggs and other organizers of the five-day salute to the veterans of the nation's most divisive conflict since the Civil War predicted that 250,000 people would attend the dedication ceremonies at 2:30 p.m. Saturday, with many of them veterans traveling here from across the country.

The 32-year-old Scruggs, an infantryman in Vietnam who was injured and hospitalized for two months, said that veterans in Georgia and New Mexico have chartered flights to Washington for the festivities, while busloads are coming from South Dakota, Wisconsin, Texas, Florida, Pennsylvania, New Jersey and other points.

He said that 15,000 people would march in the 10 a.m. parade, which will begin at Seventh Street and Constitution Avenue NW and end at 17th Street. The large majority of the marchers will be Vietnam veterans, with 800 to 1,300 veterans coming from each of the 50 states.

There also will be various color guards in the parade and 13 floats, most of them sponsored by various veterans' groups.

"I think it's going to be a very emotional time for Vietnam veterans," Scruggs said. "We've been freeze-dried now for a long time. This is the big welcome home we've all wanted.

"This is a great celebration of the fact the country has finally come to grips with recognizing the Vietnam veterans," he added. "There's been some healing."

Organizers expect that those coming to Washington will be a mixture of veterans and relatives of the war dead and missing.

"Each little town is doing it's own thing" across the country, said Pat Pellerin, a spokesman for the memorial fund.

He said veterans staged a walkathon in Portland, Ore., to raise money to send some veterans to Washington for the ceremonies.

Meanwhile, he said a veteran in Kansas City posted billboard signs and a Texas veteran distributed bumper stickers promoting the activities here. Pellerin said 11 Vietnam veterans from the Northern Marianas in the Micronesian chain of islands plan to march in the parade.

One of the ideas behind the activities is to give Vietnam veterans a chance to reminisce with each other.

Some units have reserved rooms at hotels in the Washington area for reunions, while other veterans no doubt will be discovering each other throughout the city in the next few days.

The five days of activities start at 10 a.m. tomorrow with a 56-hour, around-the-clock candlelight vigil at the Washington Cathedral in which some veterans, parents and friends of those who served in Vietnam and a handful of congressmen will read the names of those who were killed in Vietnam or are still missing.

Two volunteer readers will alternate reading names during half-hour stints and on the quarter hour a Catholic, Protestant, Jewish or Unitarian prayer will be read. The vigil is expected to last until midnight Friday.

An Entertainers' Salute to Vietnam Veterans is scheduled at 8 p.m. Wednesday in Constitution Hall. The show includes actor Jimmy Stewart and singer Wayne Newton.

On Thursday, Veterans Day, the Vietnam Veterans of America is sponsoring an all-day workshop at the Dupont Plaza Hotel on a variety of topics related to Vietnam veterans, including how to win federal assistance grants and the problems faced by women veterans.

The annual Veterans Day program is planned for 11 a.m. at Arlington Cemetery with the traditional wreath-laying ceremony.

President Reagan has been invited to speak, but a Veterans Administration spokesman said yesterday it is not yet known whether he will.

Several children of men and women who were killed in the war or are still missing are scheduled to place flowers on the Vietnam Veterans Memorial at 1 p.m. The National Red Cross has planned a reception for former Vietnam prisoners of war at 3 p.m. at its 17th Street NW headquarters. In the evening, the U.S. Army Band will play a free concert at 8 p.m. in the Departmental Auditorium between 12th and 14th streets on Constitution Avenue NW.

Seven veterans groups, as well as the Red Cross and the Stars and Stripes newspaper, are sponsoring a variety of receptions on Friday.

In addition, the Vietnam Veterans of America and the 34-member Vietnam Veterans in Congress Caucus have scheduled a 10 a.m. panel discussion about the effect of the herbicide Agent Orange on veterans and a 1:30 p.m. debate on the effect of post-traumatic stress disorders on veterans. Both panels will be held in Room 345 of the Cannon House Office Building.

On Saturday, traffic will be blocked around 9 a.m. from crossing Constitution Avenue between Seventh and 17th streets in preparation for the parade, the National Park Service said. In addition to the marchers and floats, the parade will have 23 grand marshals, including several Medal of Honor winners and Rocky Bleier, a Vietnam veteran and later a running back for the Pittsburgh Steelers. In a scene reminiscent of the war years, Air Force and Navy F4s and Army helicopters will fly over Independence Avenue at noon.

Memorial organizers say there will be a special seating area for handicapped persons to watch the memorial dedication. But for everyone else it will be standing room only and on a first come-first served basis.

The salute concludes Sunday morning with an 11 a.m. Eucharist service at Washington Cathedral.