In the story "the open window" why, do you think, the author chooses to tell the reader more about framton’s thoughts than about what vera is thinking? how would the story be different if the reader knew vera’s thoughts during the story?

Both characters are flawed.
Vera lies like a broken down barn. Her main purpose is to invent a horror story that Framton will respond negatively to. She does this "just for fun."

She reveals herself not by thought or action. What she says shows she's cruel, unfeeling, manipulative and is a thoroughly detestable person. We don't need anything more from her. There is only one way that she needs to be developed. (This is a short story after all). Her speech and her imagination do it all in a very short period of time. 

I am not taken in that she is 15 or so. Her mother is apparently a very stupid woman who has never caught on to what Vera does.

Framton has to be shown to be gullible and self centered. If he is not, he would not respond as he did at the end. What thinking adult would be taken in by a ghost story, even if it is a clever one.

No Vera does not need development, nor does Framton.