Before the time of the sage Vyasa, people had a very different understanding of which gods were important than the understanding we have today. The Vedic mantras had not yet been compiled into the four books we call the Vedas, but the mantras were still known among men, and they unfortunately led to a skewed perspective on things. Most of the Vedic hymns were addressed to gods like Indra, Agni, Vayu, Surya, Chandra, etc., because they were the most relevant to the Soma Yagna, but people got the impression that those were the supreme gods. It's only after Vyasa composed the Mahabharata and the Puranas that people got a correct view of the importance of Brahma, Vishnu, Shiva, etc. Until then, they were often viewed as minor deities, and people didn't even realize that e.g. Matsya, Kurma, and Varaha were incarnations of Vishnu.
In any case, it's interesting to trace the history of Vaishnavism (Vishnu-worship) before Vyasa came and cleared things up. Vaishnavism came from many different strands. First of all, there is Vedic Vishnuism, the worship of Vishnu as he's described in the Vedas, which mostly involves his incarnation as Vamana the dwarf. Then there is the Pancharatra movement, which began with the sage Narayana, an ancient incarnation of Vishnu. As I discuss in this answer, apparently Narayana conducted a five-day (Pancharatra) Yagna and then became the entire universe, so people started following Pancharatra texts which gave detailed procedures to worship Narayana. Then there is the Bhagavata movement, which worshipped four forms of god, Vasudeva, Sankarshana, Pradyumna, and Airuddha. Finally there is Krishnaism, the worship of Krishna due to the various miracles he had performed in his time on Earth.
The Bhagavatas and Panchartras eventually realized that Narayana was just an incarnation of the Bhagavata deity Vasudeva, so the two movements merged into one, and then the Bhagavatas realized that it wasn't a coinicdence that the four forms of god they worshipped shared the names of Krishna and his family members. And then finally, when people realized that Krishna is an incarnation of Vishnu, Vaishnavism as we understand it today was born, since people now knew that Narayana, Vasudeva, and Vishnu were all the same god; the Mahanarayana Upanishad summarizes this insight in its famous verse, "Om nArAyaNAya vidmahE, vAsudevAya dhImahi, tanno viShNu prachodayAt".
Unfortunately, we don't have any Pancharatra scriptures dating back to before the unification with the Bhagavatas. But we do have Pancharatra texts dating back to the time when Pancharatras and Bhagavatas had become one, but before Krishna had entered into the mix. The oldest of these is the Satvata Samhits, which I think was authored by Narayana himself. So my question is, is the Satvata Samhita available online in English? There is a hard copy translation here, but is this translation online? It would also be nice to have translations of three other Pancharatra texts almost as old, the Paushkara Samhita, the Ahirbudhnya Samhita, and the Jayakhya Samhita.
Note that this is a completely different text from the Satvata Tantra, which you can read here; that is a later Vaishnava text.