We are told lots of information about Mr. Framton Nuttel in the first few paragraphs that explain his susceptibility to Vera's storytelling genius. We know that he is visiting the countryside as part of a "rural retreat" because he is trying to undergo a "nerve cure". He is only visiting the Sappleton family because his sister has insisted on arranging invitations for him:
"I know how it will be," his sister had said when he was preparing to migrate to this rural retreat; "you will bury yourself down there and not speak to a living soul, and your nerves will be worse than ever from moping. I shall just give you letters of introduction to all the people I know there. Some of them, as far as I can remember, were quite nice."
Clearly, although we are given no details about the specifics of his "nervous condition", Framton Nuttel is a jittery, nervous and shy character. What is key to appreciate is that Vera is able to read him very accurately and very quickly, assessing his susceptibility to her story telling. In a sense, she establishes that he is like an "open window", in that he is so gullible that she can feel free to have some fun with him. It is this that makes him vulnerable to someone like Vera.