For vB5? There are none. This site here would be your best bet, but:
Articles on vB5:
https://vborg.vbsupport.ru/forumdisplay.php?f=249
vs.
Articles on vB4:
https://vborg.vbsupport.ru/forumdisplay.php?f=242
There is no official documentation. If you know PHP and stuff, you might want to download some existing addons and look at their code to see how they do things.