In Vivaldi's day orphanages offered intensive training in music. Music gave the young people something constructive and creative to do, especially in large groups, without requiring them to leave the grounds of the sanctuary. Music still allowed for the recognition of outstanding individuals. Concerts at orphanages, too, were well attended by influential citizens. This encouraged increased charitable contributions on behalf of orphans and led to career opportunities for young musicians who had distinguished themselves. Europe's leading virtuosi were often orphans who had been trained in such programs. The original meaning of the word conservatory, in fact, was 'orphanage.' The François Girard film Le Violon rouge (The Red Violin) offers a glimpse into this fascinating world. Those familiar with the story will recall the episode of the young Kaspar Weiss, an orphanage resident who shows exceptional talent as a violinist and appears destined for a major career.
Given this environment it comes as no surprise that Vivaldi is best known for his concertos. The Venetian orphanage boasted some formidably talented players who, by their teen years, were also exceptionally well trained. It is for these soloists, often violinists, that Vivaldi wrote these pieces. Of course, he had ample opportunity to write vocal music as well, both for soloists and choral forces. Anyone familiar with his Gloria can attest to his gift in writing for the human voice. It is perhaps more surprising that The Red Priest enjoyed writing operas as well, but in this field he had many more opportunities than one might expect for a cleric. Because the orphanage's 'graduates' often moved on to professional careers, Vivaldi in time had connections in opera houses all over Europe. His music was also well known, as Haydn's was later, though publishing.
The orphanage had demanded of its chief musician the production of new work on a constant basis. Vivaldi took pride in his ability to meet this demand, and he still stands as one of the most prolific of composers. Brittanica offers this thumbnail perspective on The Red Priest's vast body of work:
His concertos were highly influential in setting the genre's three-movement (fast-slow-fast) form, with a returning theme (ritornello) for the larger group set off by contrasting material for the soloists, and he popularized effects such as pizzicato and muting. His L'estro armonico (1711), a collection of concerti grossi, attracted international attention. His La stravaganza (c. 1714) was eagerly awaited, as were its successors, including The Four Seasons (1725). In all he wrote more than 500 concertos. His most popular sacred vocal work is the Gloria (1708). Though often accused of repeating himself, Vivaldi was in fact highly imaginative, and his works exercised a strong influence on Johann Sebastian Bach.________
'Antonio Vivaldi.' Britannica Concise Encyclopedia. Encyclopædia Britannica, Inc., 2006. Answers.com 04 Mar. 2007.