I wonder whether your question points to a bigger reality about writers and the works that they create. There is a sense in which everything they write about is a fictional construct, as much as they may try or not to imitate reality in their writings. Let us consider the following example to demonstrate this. Dickens, after his first trip to the United States, apparently came back highly disappointed from what he had seen and witnessed. For him, he had gone excited to see the new nation which was perceived as being a haven for the oppressed and an antidote for all the problems of the Old World. However, after his visit, he found that he was rather dissatisfied with what he had seen.
He wrote to his friend William Macready that "this is not the republic I came to see; this is not the republic of my imagination." This quote is highly significant because it establishes the way in which all writers, Dickens and Vidal included, already have a picture of a location or country in their mind that exists in their imagination. Their writings are the vehicle and medium through which they are able to give their imagination life and voice. Therefore we can definitely argue that Dickens and Vidal create America as a fictional construct in their work.