I searched all day for this one and still couldn't find it.....this is all i came up with He died from a heart attack.... i don't know if it's true but...oh well.
Vivaldi died , on the night between 27 and July 28, 1741 http://wiki.answers.com/Q/Special:Wysiwyg?tid=wysiwyg#cite_note-day_of_death-3, of internal infection