Ask whether the many Hindu deities are distinct beings, aspects of a single reality, or both, and you are asking the question that divides the Vedanta schools from one another. The atlas records the positions and does not choose among them.
Advaita Vedanta, associated with Shankara, holds that there is finally only brahman, one without a second, and that the individual self is not different from it. On this view the personal gods, Vishnu included, belong to the level of conventional truth: real enough to be worshipped and genuinely useful, but not ultimate. The famous Upanishadic sentences, tat tvam asi, that thou art, and aham brahmasmi, I am brahman, are read as literal statements of identity.
Vishishtadvaita, associated with Ramanuja, holds qualified non-dualism. There is one reality, but it has real internal distinctions: selves and matter form the body of God, genuinely dependent on him and genuinely not identical with him. On this view Vishnu is not a provisional form of an impersonal absolute; he is the absolute, and he has attributes, and devotion to him is not a lower path to be discarded.
Dvaita, associated with Madhva, holds outright dualism. God, selves and matter are permanently distinct, the difference between the soul and God is real and never dissolves, and liberation is not merger. Madhva's system also distinguishes souls by their natures in a way the others do not.
All three read the same texts, and each argues its position from the Upanishads, the Gita and the Brahma Sutras. A Hindu today may hold any of them, or hold none articulately and worship anyway. What cannot be said accurately is that Hinduism has a settled answer, or that the polytheistic and monistic readings are alternative Western descriptions of it; both are internal Hindu positions with long argued histories.