Very little is known about Vivaldi's personal life.  We know about his professional career through the records of the places his music was performed.  We know what people thought of Vivaldi from letters they wrote praising or denouncing him.  What we know from Vivaldi's own mouth (or hand) is minimal.  His personal opinions or thoughts are next to impossible to determine, as are the events that have nothing to do with music; romance for instance (only a step away from music.)  Vivaldi did not marry, and it is not known if he had romantic relations of any kind.  This has not prevented musicologists (wishful thinkers that they are) from speculation.

The strongest case is made for his  star opera soprano Anna Girņ, or Anna Giraud, who played the lead in his operas starting with Farnace.  She was 16 or 17 years old and Vivaldi was already 48.  She and her sister Paolina lived at Vivaldi's house and became his traveling partners, accompanying him on his excursions all over Europe for many years.  Vivaldi claimed the sisters provided much needed health care for the ailing composer, whose asthma severly hindered his priestly duties (without, apparently, affecting his abilities to travel, perform, teach, compose prolifically, or manage an opera house.)    It is no wonder that historians were putting two and two together to make love.

The first attacks against Vivaldi's lifestyle came in 1737, when Guido Bentivoglio refused to allow Vivaldi to stage an opera in the city of Ferrara.  He claimed that Vivaldi was unfit for such high an honor because he did not say mass (which was true) and that he was having an affair with Anna (which was hearsay.)  Vivaldi protested in a long rambling letter which is the only hard evidence we have of Vivaldi's involvement with the singer.  He denied the accusations, of course.  (Vivaldi eventually got an opera into Ferrara, but is was a commercial failure.)

Whether Vivaldi was intimate with Anna does not deter from their interesting relationship.  He was more than likely a father-figure and mentor, and she a pupil and close friend.  Their travels may have been strictly business, or Vivaldi may have acted as her chaperone.  She was not regarded as an outstanding singer or object of beauty, but she was known to be a convincing actor with a strong presence, and she received several compliments and denouncements in surviving accounts.  She was married seven years after Vivaldi's death.  We more than likely will never know exact details of their alliance, which means the historians (and I) are free to speculate.