This is a great question to consider, because really, there are two ways of answering this. Firstly, we can say that Framton Nuttel is the perfect victim for Vera's ingenuity and skill in storytelling because of his unspecified nervous condition. We are told that he is undergoing a "nerve cure" which makes us think that Framton Nuttel is a rather delicate and sickly individual. This is something that clearly Vera is able to ascertain very quickly when she is left alone with Framton Nuttel, and thus she is able to recognise that he would be rather susceptible to her arts. This of course makes him an excellent victim.
However, to take another angle on this question for one moment, we could argue that Framton Nuttel is a very bad choice of victim, because his nervous condition means that he is going to be profoundly and adversely impacted by hearing and believing Vera's stories. This makes him very unsuitable, because the susceptibility that makes him likely to believe her also indicates that he is going to be profoundly traumatised by the stories, which is shown by the swift way in which he leaves the household when the "ghosts" return:
Framton grabbed wildly at his stick and hat; the hall door, the gravel drive, and the front gate were dimly noted stages in his headlong retreat. A cyclist coming along the road had ot run into the hedge to avoid imminent collision.
Such a strong reaction should make us think that actually Framton Nuttel is a bit too suitable a victim for Vera's storytelling, and that what she does is actually rather cruel.