How did an incident involving a vision of Vitthal demonstrate Sai Baba's omniscience to Saheb Dixit?

Chapter 4 describes a profound experience that showcased Baba's omniscience to Saheb Dixit. After his morning bath, Dixit had a meditative vision of Vitthal. Later that day, when he went for Baba's darshan, Baba astonished him by asking, "Vitthal Patil had come, right? Did you meet him?" Baba then cryptically added that this "Vitthal" is a runaway who must be nailed down. The experience was further validated when a picture seller arrived in the afternoon with images of Vitthal of Pandharpur. The form in the picture was an exact replica of the one Dixit had seen in his vision, causing him to marvel at Baba's words and his all-knowing nature.