Nelson Vails, former New York City bicycle messenger and now one of the best sprinters in amateur bicycle racing, will ride in today's $20,000 National Capital Open bike race on the Ellipse.

With three times the $7,000 in prizes offered last year, the 20-year-old open is now one of the richest amateur races in the country. It has attracted nearly 400 racers this year, from juniors and women cyclists to an international field of 120 top amateur and professional riders competing in the featured 50 kilometers that start at 2:30 p.m.

One of the favorites in the men's main event is Canada's best amateur rider, Steve Bauer, who won last year's open and finished fifth overall in the three-day Tour of America race last weekend, ahead of 70 of the world's top professional and amateur cyclists.

Vails, the 22-year-old Harlem racer who rides for the Murray amateur team, has been training with the U.S. national team in preparation for this summer's Pan-American races and the 1984 Olympics.

Several strong U.S. amateur and professional cyclists in the 284-mile Tour of America will compete in the open, including pros John Eustice of Philadelphia and Paul Pearson of Bethesda, and young U.S. pros Jeff Rutter and Gary Doering.

Bruce Donaghy, the U.S. amateur who won the open in 1980 and 1981, will return, as will three of the Washington-area's outstanding older riders, Jim Montgomery and Rob Lea, who both hold national veterans (35 and over) championship titles, and Jerry Nugent, U.S. national masters (45 and over) champion.

Canadian Karen Strong-Hearth, who won the open several years ago, and Sue Leggett-Pearson (Paul Pearson's wife) are among the entrants in the 35-kilometer women's race, which begins at 12:30 p.m.

The veterans' race is the first, at 9:30 a.m., followed by juniors at 10:30 and senior III riders at 11:30.