I am really curious about Vivaldi RV 440, RV 418 and RV 63 (La Follia) etc:
When were they created?
Are there any certain subjects/reasons behind each composition?
What databases can I use to find information like this?
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Bad news first: If there is such a data base, I've not found it yet.
But there is some hope: Buy a CD recording and take a look into the booklet. If it is not an extremely cheap series, chances are good, that you find the mentioned information there. (Especially for highly productive composers the year of composition or any background may not be known in general, however). Some classical labels even offer their booklets for download free of charge. Especially for exotic composers the booklet is the most promising approach, since the artist often wants to share his or her discovery, while guides (see below) may consider the composer as "not sufficiently important".
Musical guides also exist as books and may cover an era, a single composer or a genre like chamber music.