Warren Vache: ''Iridescence.'' Warren Vache, comet, flugelhorn; George Duvivier, bass; Hank Jones, piano; Alan Dawson, drums. Planet P-24.--The rhythm section is the thing on this album. Not to slight Vache's fine playing, but the combination of Duviver, Jones, and Dawson would be dynamite backup for any horn player. Dawson is certainly Boston's finest and most sensitive drummer, Hank Jones is one of the most inventive of all jazz pianists, and Duvivier is a solid time player and a melodic soloist (check out Duvivier's solo on ''Autumn in New York.'':. Vacge, who says he chose comet over trumpet because of its versatility, plays at times with a sweet growl, and other times a full-bodied, fat and brassy tone. He has a good grasp of dynamics, and he fits with this rhythm section like the proverbial hand in a glove. He's obviously urged on and inspried by their creativity. The mood is relaxed and easy, and the set abounds with medium tempos--not the kind of excitement that will jar you oyut of your seat, but nonetheless gripping in ito own way. Hank Jones has contributed the lush balad that became the title of the album, 'Iridescence.''