A chief characteristic of Vera—and the one that drives the story—is malice, which is the trait of doing evil for pleasure. Vera tells elaborate lies for the sense of enjoyment it gives her. This goes along with her cruel streak: when she finds out, for example, that Nuttel is suffering from a nervous disorder, she does her best through her lying to exploit that weakness in him, play on it, and drive him further into anxiety—so much so that he flees the house in terror.
Vera seems to have a strong desire for power, which perhaps reflects her sense of a being a powerless adolescent in her aunt's house. She gains power and control over others through telling lies. She can feel superior to those around her through her lying, because it leaves her knowing truths they don't and playing them for fools.
Vera's behavior shows she is passive aggressive. Her upbringing as a lady with good manners means she can't outwardly say that she finds Mr. Nuttel a bore who has been thrust on her or her aunt and uncle a fool, so she manipulates them using lies as her outlet.
We like Vera, however, on some level, because she poised, creative, intelligent, quick-witted, expert at her ploys, and very daring and audacious—perhaps the epitome of the artist. Our shock comes in the slippage between our expectations of how a well-brought up and polite teenage girl should act and her malicious behavior.