Vaquero refers to generally to a "cowboy" but more specifically to a herdsman or cattleman. The vaqueros of Antonion's father's generation moved cattle from place to place across the vast open spaces of Texas and the wider southwest.
The term for those open spaces is "llano", which means planes.
Antonio's father longs always to return to the type of freedom he associates with herding cattle across the southwestern planes. He feels that in movement there is freedom and fulfillment. However, after seeing his three oldest sons claimed by that desire for movement and freedom - and losing them to that desire, Gabriel begins to feel differently about these impulses.
". . . Every generation, every man is a part of his past. He cannot escape it, but he may reform the old materials, make something new."
In Bless Me, Ultima the Ilano is basically a plain and the Vaquero a cattleman or herder in the traditional Mexican community. The two words are important because they represent the conflict facing the protagonist in the story. To attain maturity Antonio is taken through a mystic journey by Ultima, the curandera. Antonio’s mother who is from a farming family wishes for his son to become a priest in the farming society but his father wants him to take up the Vaquero lifestyle and herd in the Ilano. According to his father, being a herder would be in keeping with his family’s tradition and culture. This immediate conflict in the family is replicated in Antonio’s spiritual life, where he is faced with differences between his catholic religion and the information he encounters in his mystical journey. Ultima guides Antonio and offers him an opportunity to understand the conflicts he has encountered. The benefit of this journey is to help him find his own path.
A llano is a plain and a vaquero is a cowboy.