Donna Pratt of San Diego made it through the hour-long ceremony without crying yesterday at the Vietnam Veterans Memorial service honoring the 110 men whose names were recently added to the black granite wall.

But when the official ceremony was over, the speeches finished, the names read out loud, Pratt burst into tears when talking about her husband, James I. Pratt, who was killed in 1965 in a plane crash off the coast of Vietnam.

"He is finally getting what he should have gotten long ago," she said, echoing the sentiment of many of the more than 400 family members who attended the Memorial Day ceremony yesterday afternoon. "I think he can rest peacefully now."

U.S. Park Police estimated that 12,000 people had come to stand behind temporary fences and hear the U.S. Army Band play, watch the presentation of the 60 flags representing states and territories and listen to a few speeches.

The families of the 110 men whose names were added last month were seated together between the fence and the memorial.

President Reagan, in a speech earlier in the day at Arlington National Cemetery, spoke to a crowd of 6,000 about the Vietnam veterans.

"They were quite a group -- the boys of Vietnam -- boys who fought a terrible and vicious war without enough support from home; boys who were dodging bullets while we debated the efficacy of the battle," he said. "They learned to rely on each other."

At the memorial on the Mall, John Wheeler, chairman of the Vietnam Veterans Memorial Fund, spoke about the children who visit the wall daily.

"Each day, 2,000 kids visit this memorial," he said in a brief speech. "I listen to the questions that the kids who visit here ask and they always ask the hardest question, 'Why did these men die?' "

"I tell them they died because they kept the promise to be soldiers for America and because they wanted to help other people," Wheeler said.

"And they died because, in their heart, they hoped and they had faith that America was sending us to help Vietnam to be free."

Politely answering questions while his parents and sisters listened, 14-year-old Herb Stober of Waukesha, Wis., said he wrote to Reagan and his congressman asking that the name of his uncle, Herb Stober, be added to the memorial. "No one ever wrote back to me," he said. "But I kept hoping. I felt confident that his name would get in."

According to other family members, Herb Stober was killed in Thailand in 1969 when bombs he was loading onto a forklift exploded.

The new names are those of 97 servicemen who died during the war but outside the war zone, and of 13 who later died of wounds received in Vietnam. They were added to the 58,022 men and women in the military who had died in the war zone and whose names appeared on the memorial when it was opened in 1982.

Ford Motor Co. underwrote the cost of the new inscriptions plus paid the expenses of the search committee that located and invited the families being honored yesterday. Ford's Chairman Donald E. Petersen drew a standing ovation when introduced to speak.

Except for a few veterans of various wars who attended yesterday's ceremonies in uniform, the crowd was decidedly casual in dress. Women wore sundresses and children wore shorts. Many men were jacketless.

Donna Pratt's son Eric, 23, videotaped yesterday's ceremony to show to a few relatives who did not come from California for the event.

"It wouldn't be right for his name not to be on the wall," he said of his father. "He loved his country and that is why his name is there today."

Wendi Stinnett, 13, came with her family from Tennessee because the name of her uncle, Gordon Wheeler, who was killed in 1969 when his plane crashed in Thailand, had been added to the wall.

"I didn't know him, but I am glad his name is up there," she said. "He fought and he died and he deserves to be there. Next year, I am looking forward to talking about the Vietnam War in class."

Jim Stober, Herb Stober's brother, came to the ceremony wearing his Air Force Reserve uniform. Both served in Vietnam.

"I always recognized a certain element of risk for myself but I always thought my brother would make it," he said. "It has weighed heavily on my heart ever since.

"We were shocked when we discovered that Herb's name wasn't on the wall," he said. "What bothered me was that he just wasn't in the right location. It could have been me crashing in Thailand. It just didn't jell the way they drew the lines."

Stover said he felt it "heartwarming" that the bureaucracy in Washington was finally able to correct the earlier oversight and add the new names to the wall.

But JoAnn Sharkey of Palmdale, Calif., sister of James Pratt, did not find much comfort in the memorial or the ceremony.

"We are a very close family and Jim's death has left a big hole in our family," she said as she sobbed into a handkerchief. "Two children have been raised without a father and there is nothing that anyone can do about that now. They can add his name to this wall but that doesn't bring him back. I am never returning to this wall, ever again."