The observation that many people claim that Vishnu exists is evidence against the proposition that there is no god but Allah. But the observation that many people claim that there is no god but Allah is evidence against the proposition that Vishnu exists.

We could try to factor this as "a divine being exists" vs. "that divine being has properties X, Y, and Z", but different traditions don't even agree on what a divine being is. Mormonism and some Dharmic traditions, for instance, consider that humans could be reinstanced as divine beings after death, whereas most Abrahamic traditions do not.