Today my grandfather told me that our Kuldevta is Lord Vishnu and I had a hard time believing it. Is it possible to have Vishnu as a Kuldevta? If yes then which Rishi was originated from Lord Vishnu?
Yes, this is absolutely possible.
A famous example: The kuladevata of the Travancore royal family was Padmanābha, Lord Vishnu. The kings from this house even carried the title "Padmanābhadāsa", servant of Vishnu. They considered Lord Vishnu to be the true ruler of the country and themselves to be mere custodians.
The kuladevata of most Iyengar brahmins (who belong to the Sri Vaishnava tradition) is Vishnu.