For Washington listeners who love the human voice, the song recital series presented by Vocal Arts D.C. offers the most refined delights. Since its founding as the Vocal Arts Society of Washington twenty years ago, the organization’s genial director, Gerald Perman, has presided over a series of triumphs: exquisite concerts of art songs and opera by the world’s best singers, some of them already known and others given their Washington debuts. To celebrate, the group joined with Washington Performing Arts Society to host Joyce DiDonato for a praiseworthy concert in the Kennedy Center Concert Hall last night.