To what extent is Vera the mastermind in "The Open Window"?"The Open Window" by Saki
Saki's "The Open Window" is a framed story. As such, Vera is in control of the tall-tale that is within the story of Framton Nuttel's visit to the countryside. This tale, which controls most of the narrative, is in response to the weak, neurotic adult that the precocious child finds tedious. She preys upon his susceptibility as well as his ignorance of the Stappleton family and the area by fabricating a tragedy that has not happened. Thus, Vera completely dominates the adult in her presence. Even when the husband and brothers of Mrs. Stappleton return, the undaunted Vera feigns horror in order to terrify the weak Framton Nuttel. To the very end, the powerful, clever child completely dominates the adult world, as with swiftness she responds to her aunt's wonderment at Framton's frantic fleeing from their house with yet another fabrication.
I think that it is completely clear that Vera is the mastermind who is driving this story. No one else really plays any sort of an important role in causing the events in this story to happen.
Framton Nuttel drives the situation by coming to Vera's home, but beyond that, he is not in charge. Only Vera is in charge. She is the one who is going to decide what stories she will tell. The stories she tells (first to Nuttel and then to her aunt) make the story what it is. No one tells her to make up these stories, so no one else is controlling what happens in this story.
So Vera is, without doubt, the mastermind in the story.