What does Vera in the narrative of "The Open Window" do that proves she is confident?
Vera's "romance at short notice" proves that she is quite confident. That is, her glibness and fearlessness in her art of fabrication demonstrates her confidence that people will believe her.
While Framton Nuttel waits upon her aunt's arrival to receive him, Vera--with great bravado and opportunism--composes a fictional tale about her aunt's husband and her two younger brothers that has them suffering loss of lives. She weaves such a compelling story of their having been swallowed up by a treacherous bog that the timid Nuttel is quite moved. When the men do actually return, Vera has woven such a realistic and clever tale that these men appear to have been resurrected from the dead to the impressionable Nuttel when he hears Mrs. Stappleton cry, "Here they are at last!"
But, Vera's credibility is not shaken because she has prepared for the inevitable. She has told Nuttle,
"Sometimes on quiet evenings like this, I almost get a creepy feeling that they will walk through that window--"
When this action occurs, the undaunted Vera acts horrified. In "nameless fear," Framton flees. The male members of the family greet Mrs. Stappleton, and Mr. Stappleton inquires as to who it was who fled so quickly. But, when Mrs. Stappleton expresses equal wonder, Vera calmly replies with yet another lie,
"I expect it was the spaniel....he told me he had a horror of dogs. He was once hunted into a cemetery somewhere on the banks of the Ganges, by a pack of pariah dogs....enough to make anyone lose his nerves."
Vera never gives up and is very clever.