Like any other character in any work of fiction, Vera is the kind of person she is because her creator, Saki, made her that way. He wanted to write a story in which an extremely nervous man is persuaded that three live humans are ghosts. He could have chosen any kind of character to tell about the three men who were sucked into a bog.
Saki may have considered using an eccentric old butler or a senile relative, but he hit on the perfect character with Vera. She has to be old enough to be taken seriously, but not too old for such mischief. In Saki's day a girl like Vera would lead a life of boredom. Being a girl, she couldn't go out hunting. She probably spent most of her time reading books and indulging in fantasies. One of her fantasies might have been that the three hunters would get sucked into a bog on one of their stupid hunting excursions.
Vera is not only bored but resentful. She hates being stuck in this house with her brain-dead aunt and the three men who have nothing to talk about but killing birds. Then when forced into playing hostess to another totally uninteresting man, she takes some of her feelings out on him. She doesn't like him. She wouldn't mind frightening him so badly that he would leave. She draws on her fantasies and her reading of lurid novels by writers like H. Ryder Haggard to make up a story.
Vera is exceptionally intelligent--which also explains why she is so bored with the life she is forced to lead as a housebound girl in the Victorian era. She is so familiar with the daily routine that she knows when the men will be returning and what her aunt will be talking about while she awaits their arrival. Vera has seen the same things and heard the same words over and over until she can predict precisely what will be seen and heard in a short while. The fact that it is tea time in an English country manor determines the timing. The spirit of the story is something like the excellent movie Groundhog Day (1993), starring Bill Murray. Exactly the same things keep happening every day at exactly the same time. Like the character played by Bill Murray, Vera tries to shake things up a little.
Vera is not a much better actress than many other teenage girls. She is described as a very self-possessed young lady. This is not an act but her real self. The acting is reserved for the time when the three hunters appear.
In the deepening twilight three figures were walking across the lawn towards the window; they all carried guns under their arms, and one of them was additionally burdened with a white coat hung over his shoulders. A tired brown spaniel kept close at their heels. Noiselessly they neared the house, and then a hoarse young voice chanted out of the dusk: "I said, Bertie, why do you bound?"
All of this description is intended to assure the reader, as well as Framton Nuttel, that these must be the three men who are supposed to be dead. But Framton is frightened by Vera's act of terror before he turns and sees these figures approaching. When Mrs. Sappleton says, "Here they are at last!",
Framton shivered slightly and turned towards the niece with a look intended to convey sympathetic comprehension. The child was staring out through the open window with dazed horror in her eyes.
It is the contrast between Vera's "self-possession" and her change to a "child" with dazed horror in her eyes that unnerves poor Framton Nuttel and prepares him to see the worst when he turns towards the open window.