THE 29th VASSAR Book Sale will open at 10 a.m. in downtown Washington at the end of April - or maybe the begining of May. As usual, nobody knows exactly where or when. "We're waiting to find a rent-free, empty building," says Chairman Sally Roman, Vassar '65. Then, 100,000 volumes will be trucked to the site, and the five-day extravaganza will begin. The first Vassar Book Sale, held in 1949 at St. John's Church (Lafayette Square), earned $500. Last year enough books were sold to send 20 students to Vassar on scholarships.

Meanwhile, in their basement collection center near the National Zoo, crews of old grads have been sorting, pricing and boxing the books which have been cascading in since the end of last year's sale. "We're absolutely swamped," Nell Cooley, class of '23, says happily. "We have more and better books than ever before - but there can never be too many." (The American Indian, in two volumes, by George Catin - considered rare - has been priced at $75; ordinary hardbacks start at 50 cents.)

Local book addict rise at dawn - or spend the night on the sidewalk - to get first crack at the merchandise. Call 223-6274 for late-breaking developments.