Is Vera a good hostess?

The short answer is no. A good hostess would do everything she could to put poor Framton at ease, especially after finding out that he's clearly of a nervous disposition. But Vera does no such thing. Instead, she exploits the opportunity to exercise her vivid imagination, spinning one of her legendary tall tales to send a frightened Framton running for the hills.
What makes her little game all the more effective is that she appears to be such a harmless individual, a young girl from a respectable family. She plays the part of hostess rather well to be begin with, engaging Framton in small talk until Mrs. Sappleton arrives. But that's just it: Vera's playing a part. She's not really being a good hostess. It's all just a front of formal politeness, which lulls Framton into a false sense of security, making it easier for Vera to play her cruel prank later on.

Vera is not a good hostess. A good hostess makes her guests feel comfortable and welcome. Vera knows her aunt's guest, Mr. Nuttel, has a nerve disease and is in the country for a rest cure, but she nevertheless plays on his fears with a false story about ghosts coming through an open window. She manipulates events to drive Mr. Nuttel away.
Of course, while she is a poor hostess, Vera may have done a good turn for her aunt. It seems clear that Mr. Nuttel is a bore who goes on too much about his symptoms and nerve troubles. We learn that he "could only talk about his illnesses." Vera's aunt, Mrs. Sappleton, seems to delay her entry so as to avoid him, and when he begins to talk about diet, she can hardly contain her boredom:

"No?" said Mrs. Sappleton, in a voice which only replaced a yawn at the last moment.

Vera's deliberately poor hostessing skills may have saved the family from an unwanted guest.

Saki's story opens with the following dialogue:
"My aunt will be down presently, Mr. Nuttel," said a very self-possessed young lady of fifteen; "in the meantime you must try and put up with me."
It is ironic that Vera is serving as a hostess when her intention is to do just the opposite of what a hostess should do. She intends to make the guest uncomfortable rather than comfortable, and she succeeds possibly beyond her own expectations. We might wonder why this girl is behaving in such a perverse way. On the surface she is poised and polite, but there must be a lot going on beneath that smooth surface. Vera seems to be bored and rebellious. She creates her ghost story just because she wants to do the opposite of what she is supposed to do. Her aunt is probably trying to teach her to be a good hostess. This could explain why Mrs. Sappleton isn't there to greet Framton Nuttel. She sends Vera on ahead so that this girl, who will soon be of marriageable age, can get some practice playing the same sort of role that her aunt has been playing for many years. But perhaps Vera doesn't want to be like her aunt. Her decision to make up a story about the three hunters being sucked into a bog indicates how terribly bored she must be. There is little for a fifteen-year-old girl to do in this country household. She sees the same actions and hears the same words being repeated every day until they become maddening. She hears that silly refrain repeated every evening at tea time:
"I said, Bertie, why do you bound?"
It is because Vera knows exactly what is going to happen and what is going to be said that she is able to make up a story that will frighten poor Framton out of his wits. She knows her aunt will talk about nothing but wild birds and bird-shooting because that is all the poor woman ever hears in this country home. She knows her aunt will be looking out the open French window and waiting for her menfolk to come home in their muddy boots. Mrs. Sappleton's life revolves around her husband and brothers. This is the sort of fate Vera sees in store for herself when she gets married off, and this probably explains her dominant feelings of boredom, anger, and rebelliousness. The poor girl can only rebel in her imagination. Framton provides her with a rare opportunity to vent some of her concealed, smoldering hostility.
Mrs. Sappleton probably has nothing important to do upstairs. She is just marking time while her niece has an opportunity to play hostess. Vera plays hostess-from-hell by telling the visitor a grisly story about how three men got sucked into a bog and died horrible deaths. She has just enough time to finish her story before her aunt arrives and takes over the hostess duties.
She broke off with a little shudder. It was a relief to Framton when the aunt bustled into the room with a whirl of apologies for being late in making her appearance.
Now the scene is set. All Vera has to do is sit back and wait.