Esther Vanhomrigh is known as "Swift's Vanessa" because of the fictional name he gave her when he published their correspondence. The portrait is wholly imaginary. No  image of Esther Vanhomrigh  survives and we  know very little about her actual appearance, although she was said "not to be a beauty".
Esther Vanhomrigh is referred to as "Swift's Vanessa" because Swift assigned her this pseudonym when he published their correspondence. The portrait is entirely fictional. There are no surviving images of Esther Vanhomrigh, and we have scant knowledge about her appearance; she was said not to be beautiful.