You have highlighted the last sentence of this incredibly amusing short story which contains irony on so many levels. Vera, of course, is the storyteller without equal, who is quickly able to seize on details and weave convincing tales to horrific effect. Note how she dominates the story - it begins with her words and ends with them. We are told in the first sentence that she is "a very self-possessed young lady of fifteen". It is clear that she sees in Framton Nuttel an object for one of her stories, as she is quick to establish that he knows nobody from the area and thus she is free to use her excellent wit and intelligence to create a fable that will shock Framton Nuttel for her own amusement. She shows herself to be an excellent actor as well as a storyteller. Consider how the author narrates her duping of Framton Nuttel:
Here the child's voice lost its self-possessed note and became falteringly human... She broke off with a shudder.
She is not only creative, but quick, intelligent and able to fool others into believing her words. This is demonstrated yet again at the end of the tale when, nonchalantly, she creates another tale to explain Framton Nuttel's swift escape from the house to trick her family, telling the tale "calmly" with complete equanimity. Clearly this tale celebrates the power that a good storyteller can have over a susceptible audience, with Vera presented as the master storyteller.
We can see “Romance at short notice was her speciality”. This ending means that Vera can tell great stories in a short time which also let us straightforward understand Vera is smart, cunning because she is too smart that makes him as well as cunning to use her good making story to trick Franton Nuttel so she is very cunning in the last line.