No, I don't believe in the immortality of Vyasa, or else, i'd have believed that he was around to write the Vedas. I'm open to the probability that the Puranas preceded him and others came much later - I only suggested that he wrote some
of them, which is not the same as claiming that he wrote all
All the devas, such as Indra, Surya, Varun, Vayu, Yama, et al were recognized gods, and worshipped as such, even as late as the Dwapar Yuga (the incident of Krishna & the Govardhan mountain). So calling it monotheistic is still inaccurate. And when you reference western religions, I'm assuming that you're not talking Judaism or Chirsitanity here, but rather, Greek, Roman and Norse religions, where you had Gods like Apollo (the Greek equivalent of Surya), Zeus (the Roman equivalent of Brihaspati) and Thor (the Norse equivalent of Indra). All 3 of these were polytheistic, as was the Vedic religion.
Even the trinity model in Hinduism - Brahma, Vishnu & Shiva- qualifies as polytheistic, by definition. It's not a question of whether monotheism is better than polytheism or vice versa, it's just a question of the terms. Any religion w/ just 1 god is monotheistic. Any religion w/ >1 god is polytheistic. The post-Puranic model is more pantheistic i.e. every god gets to be recognized as such and worsipped.