Given the historical condition in which the book takes place, essential vocabulary can be pulled from both literary and social contexts. On one hand, I think that "slavery" is an essential vocabulary term in chapter 1. This is the condition that describes Isabel and her sister, and helps to drive much of Isabel's actions throughout the narrative. Another important vocabulary word that emerges from the first chapter would be "smallpox," the disease that kills Isabel's mother.
Finally, the word "cemetery" is important in chapter 1. It is here in which the reader gains much insight into Isabel. It is through the cemetery where one is reminded of the socially divided worlds of White and Black. It is in the cemetery where Isabel recognizes her dilemma as someone who is fighting for freedom against overwhelming odds. Such realization triggered by "cemetery" helps to bring a point of convergence into Isabel's plight with that of the Colonists and also bring out a significant point of divergence. This duality is where her narrative becomes so compelling and the vocabulary in chapter 1 helps to enhance this.