To think of Vivaldi only in terms of the crowd-pleasing Four Seasons is to slight the more significant accomplishments in his lengthy and prolific career. L’estro armonico, his opus 3 (1711), for example, is acknowledged by musicologists as perhaps the most influential collection of instrumental work to appear during the whole of the 18th century. Most of us, no doubt, are familiar with two or three individual pieces from it, such as the Concerto in A Minor; but the dozen string concerti are so seldom performed as a set that few concertgoers have had the chance to savor the strikingly elaborate architectonics that startled Vivaldi’s contemporaries. Estro, incidentally, means heat, and these pieces, with their magnificent ritornellos and spirited solos, indeed radiate energy, passion, and a refreshing perkiness. They are just the right stuff to showcase the renewed vigor of the Chicago String Ensemble–one of a handful of string orchestras in the country. After several seasons of uneven playing, this underappreciated group is reasserting the rich, velvety sound that has reminded me of the Philadelphia Orchestra under Ormandy. Conductor Alan Heatherington will be among the happy fiddlers. Tonight, 8 PM, St. Pauls United Church of Christ, 2335 N. Orchard. Saturday, 8 PM, Hammerschmidt Chapel, Elmhurst College, 190 Prospect, Elmhurst. 332-0567.