AFAIK he wrote about 500 concerti, mainly for violin but also for far more exotic combinations. Some of them are really beautiful, but people insist that he wrote 500 times (I think the right number is well under 500 and fairly above 448) the same concert but that is wrong. Bach liked his music (the estero armonico - extremely interesting - indeed beautiful music).
He also wrote a lot of operas, and the way he managed the affairs (he was also an impresario) is well caricatured by Benedetto Marcello (Vivaldi is protrayed as Aldiviva). He particularly fancied a young girl singer that seems to have had a not very good voice (he expressely asked for not very difficult parts to be written for her) which led some biographers to infer that she was his lover.
But others claim he may have been a castrato because otherwise he wouldn't have been trusted with the musical education of a girls school.
Anyway, I don't think he lacked technique: German musicologists say that the estro harmonico concerti transcribed by Bach are more Bach than Vivaldi, but I assure you that is not the case. They are mostly mere transcriptions and the harmony is somewhat thickened - but that is normal on the organ.
Vivaldi had a marvelous talent for melodies - therefore it is only natural that he liked opera. It was also a highly rewarding activity (in terms of money) because opera was closer to the popular musical comedy than to the modern opera house - it was a popular thing, and in many ways remained that way in Italy. I mean, before someone attacks me, that it has remained a non-elitist thing (I hate Verdi, by the way)
