Nancy Reagan's mailbox will be filled Tuesday with an eight-foot valentine from crooner Bobby Vinton. Currently under construction, the valentine is made out of Gatorfoam, a plastic-covered foam, painted red and trimmed with lace, with a cupid on the top. It will read, "To the First Lady, Happy Valentine's Day. Love, Bobby Vinton."

Vinton said he sang for Reagan last year at a Los Angeles benefit in her honor. "She wrote me a very nice letter . . . I have it framed on my desk," the 50-year-old singer said. "I wanted to send her something bigger than she's ever received." The valentine will arrive three days early because Reagan will be leaving for Santa Barbara, Calif., Wednesday, and Vinton wanted her to receive it while she was at the White House. "The intent is there. I hope she can appreciate it a couple days early," Vinton said. Rewarding the Richly Written

French writer Marguerite Duras, Mexican novelist Carlos Fuentes and West Indian-born author Jamaica Kincaid were named finalists Friday for a $50,000 book award, the world's largest prize for a novel. The winner of the Ritz Paris Hemingway Award is to be announced April 7 at the Hotel Ritz in Paris, where novelist Ernest Hemingway lived for several years, said Robert Woodrum, executive director of the award. The first winner of the award was Peruvian author Mario Vargas Llosa, whose "The War of the End of the World" was named best novel of 1984.

Duras was selected for "The Lover," about a love affair between a 15-year-old French schoolgirl and her wealthy Chinese admirer. Fuentes was named a finalist for "The Old Gringo," his novel based on the life of Ambrose Bierce. Kincaid was selected as a finalist for her first novel "Annie John," about growing up in her native Antigua. The award is sponsored by Egyptian businessman Mohamed Al-Fayed, owner of the Ritz, and Harrods department store in London. Peaceful Yarn

Maine farmer Peter Hagerty has blended wool from Soviet and American sheep into "peace fleece," saying he hopes it leads to warmer, close-knit relations between the superpowers.

Hagerty said yesterday that he and his wife Marty Tracy had been searching for a way they, as a farm family, could help promote peace when they had the idea to import Soviet wool and weave it with domestic wool into yarn. Hagerty, who shears sheep on his farm in Kezar Falls, hopes Americans will buy the yarn and knit scarfs and mittens as symbols of peaceful cooperation. Grooming the Prince

It seems Prince Andrew may finally settle down. The Daily Mail, a London newspaper, said yesterday the prince is likely to become engaged in June to his girlfriend Sarah Ferguson. Andrew's mother Queen Elizabeth II reportedly has given her blessing to the engagement, and Princess Diana, a close friend of Ferguson's, was quoted as saying the couple are expected to wed this year. End Notes

Esquire, the self-proclaimed magazine of "Man at His Best," is reportedly getting ready to launch a new publication geared toward women. The new quarterly magazine, whose working title is "New York Woman," will debut in September, and Esquire Editorial Director Betsy Carter is slated for the position of editor. Carter had no comment about the project . . .

Lady Bird Johnson, who was hospitalized Wednesday, will remain in St. David's Community Hospital in Austin, Tex., at least until tomorrow, her office said. The former first lady, 73, was hospitalized for testing after a fainting episode and a fall earlier in the week in which she injured her knee.