If a Muslim worships Allah, a Jew worships Yahweh, and a Hindu worships Vishnu, can we be sure that the third is worshipping a different god from the first two?
Here’s one theory: Originally, all humans knew and worshipped the one true God, and that knowledge was passed down over the generations. Over time, however, distortions in the idea of God crept in; this process was not uniform, in some areas far greater distortions occurred than in others - in part, we may say this was due to a certain portion of humanity (the chosen people) being granted a certain protection against such distortion by the special intervention of God. If this theory is true, it would follow that Vishnu is actually a distortion of the one true God; which would imply that a person who worships Vishnu is actually worshipping Yahweh, but out of ignorance they do not realise that Vishnu is a distored account of Yahweh.
I don’t think the fact that someone is a polytheist excludes the possibility they are ignorantly worshipping the one true God; their many polytheistic gods may all be different distortions of the one true God. A person suffering from ignorance may mistakenly believe the one entity to be two different entities. A person ignorant of astronomy might believe that the Morning Star and the Evening Star are two different objects; to give a fictional example, a person not in the know would believe that Clark Kent and Superman are two different people. In the same way, a Hindu might believe that Vishnu and Shiva are two different gods, even if they are really just two different distortions of Yahweh.
We should also remember that many Hindus are not strictly polytheists: they believe all their many gods somehow constitute one single ultimate God. The same belief was common among pagan philosophers in ancient Greece and Rome.
So, on the basis of the above, I’m inclined to say that the Hindu worships the one true God, but that the Hindu’s knowledge of that God is somewhat more distorted than the Muslim’s is.
Simon