Were there any instances of devotees seeing visions of Lord Vitthal in Shirdi?

Yes, Chapter 4 describes multiple accounts of devotees experiencing visions of Lord Vitthal in Shirdi. Sai Baba assured Das Ganu that Vitthal would appear if the devotee was full of faith, and indeed, Das Ganu is said to have received the sight of Vitthal at the end of a week-long worship period (Saptah). In a separate event, Kakasaheb Dixit had a vision of Vitthal during his morning meditation. Baba later confirmed this experience by asking him, "Vitthal Patil had come, right?" As detailed in Chapter 4, this was further validated when a picture seller arrived with images that perfectly matched Dixit's vision, prompting him to purchase one for his personal worship.