How does the narrator present Vera in The Open Window?

At first, Vera's presented as a rather sweet, demure young lady. This is important because it helps to lull Framton Nuttel into a false sense of security, making Vera's subsequent prank all the more effective. Yet there are subtle hints as to what kind of person Vera really is right from the very first lines of the story:

"My aunt will be down presently, Mr. Nuttel," said a very self-possessed young lady of fifteen; "in the meantime you must try and put up with me."

We see that Vera is self-possessed, very sure of herself. Yet at the same time she acts in a self-deprecating manner, which makes it easier for her to play her cruel trick on Framton. It's all an act, but then Vera needs to be a good actress if she's going to make her horror story sound plausible. And so it goes on. After Vera has finished relating her story to an increasingly agitated Framton, she breaks off with a shudder. One gets the impression she's done this before, and is getting rather good at putting on the appropriate gestures. That shudder seems almost second nature to her.
When the three men return home from their day of hunting, Vera affects a look of sheer horror. It's not enough that she has to know how to tell a good yarn; she needs to keep up the pretence for the prank to have the maximum effect. And it does.
After a terrified Framton suddenly takes off, Vera effortlessly slips back into her role as a sweet young lady, seemingly perplexed by all the hullabaloo:

"I expect it was the spaniel," said the niece calmly; "he told me he had a horror of dogs..."

Though she may have fooled Framton and her aunt, we now know what kind of person she is:

"Romance at short notice was her speciality."

So she has indeed done this kind of thing before. Just as we suspected. That explains why she's so incredibly good at it. But the last line in the story is fascinating because it makes us wonder just how far Vera's prepared to take her penchant for telling tall stories. Is it just a harmless phase of young adulthood or does it express something more sinister about Vera's personality? We must make up our own minds.