In This Is What It Means to Say Phoenix, Arizona, why can Victor be described as a realist, while Thomas can be described as a dreamer?

Without question, there are character differences between Victor and Thomas-Builds-the-Fire.
Thomas maintains more of his Indian identity than Victor does.
Thomas-Builds-the-Fire lives on the reservation, and he tells stories; he also speaks of dreams, employs Indian phrases, and invokes legends. He seems to know things without having been told anything.
The young men cannot be warriors because, as Thomas says, "All the horses were gone;" that is, they are confined to a reservation, and there are no people against whom they can demonstrate their warrior spirit and skills.  Nevertheless, Thomas-Builds-the-Fire does not surrender to these deterrents as does Victor. For Thomas speaks of dreams; he believes in the power of dreams. When Victor's father runs off, Thomas goes to Victor and tells him in the heartfelt language of dreams,

"I heard it on the wind. I heard it from the birds. I felt it in the sunlight." Only then does he admit, "Your mother was just in here crying." 

This Indian spirit and a deep feeling for his father is lost to Victor. After they reach Phoenix, Thomas tries to rekindle it in Victor by asking him if there is anything valuable in the man's trailer. "I thought his money was in the bank," Victor replies, demonstrating his rationality. Thomas explains, "It is. I was talking about pictures and letters and stuff like that."  
Thomas extends his love and understanding toward Victor despite their no longer being friends. Victor, in turn, begins to appreciate Thomas and promises to listen to one of his stories.
As Victor sits at his kitchen table, realizing that he does not have enough money to travel to Phoenix to pick up his father's remains, Thomas-Builds-the-Fire appears at the door, saying "Ya-hey, Victor . . . I knew you'd call me." He offers to pay for the flight to Phoenix, but only on the condition that he accompany Victor.
While they travel, Thomas tells Victor of the dream he once had that led him to Spokane; he hopes that this retelling will give Victor more understanding of his father. Thomas begins by saying that his dream told him to wait for a sign. As he waited at a falls, Victor's father approached Thomas. The man asked him what he was doing there, warning him that he could get mugged. So, Victor's father buys Thomas dinner and drives him back to the reservation. After this account, Thomas tells Victor, "For a long time, I was mad because I thought my dreams had lied to me. But they hadn't. Your dad was my vision."
Once they return to the reservation, Thomas-Builds-the-Fire asks Victor only one favor. He asks Victor if he will just once stop and listen when he tells a story. Victor agrees to do so; then, Victor turns his truck toward home. Thomas closes his door and hears a new story coming to him.