The phrases "guessed vaguely" and "something unusual" affect the meaning of the sentence because These phrases emphasize the great anticipation Keller is experiencing.
Explanation:
This excerpt from "The Story of my life" by Helen Keller, presents one of her memories with expectation, these words alone give the idea of something that intrigues the person but the rest of the lines support this idea by saying that there was a lot of activity in the place so she went to wait for the thing that was coming.
