No, Guest, that's a myth. Whoever sparked that rumour probably mixed up Vivaldi with Geminiani - both he and Carolan were frequent visitors at Lord Mayo's home, which was something of a cultural centre. Incidentally, that story about Carolan's Concerto having been written in rivalry to Geminiani is also apocryphal, fun though it is. In fact it was composed for Fanny Power's mother.