Vishnu is the Supreme God in the Vaishnavite tradition of Hinduism. Smarta followers of Adi Shankara, among others, venerate Vishnu as one of the five primary forms of God.
He appears in "Screwed the Pooch" when Peter Griffin thanks Jesus for his good fortune. In heaven Jesus starts to explain that it wasn't he who did it. Vishnu stops him and says that he is accustomed to not being thanked and credited.