Could you offer some context here? Where did you get the information that the Vijayanagar Empire was Islamic in the first place?

The Vijayanagara Empire was never Islamic, it was founded by 2 Hindu's (Kanadigga or Telugu, we don't know) who were assisted by the Sringeri Guru, Vidyaranya.

The empire itself was founded as a bastion against Islamic invasions from both the north and South (the short lived Sultanate of Madurai) and along with the then dying Hosyala Kingdom resisted the invasions of various Islamic armies.