According to some writings, Vishnu (Preserver) is the underlying All and source of everything there is;
When Visnu lying in the ocean of milk willed to create this universe, a golden lotus sprang from His navel-pit. The golden lotus with its stem is the abode of Brahma representing Brahmaloka or Satyaloka. Srimad-Bhagavatam
The original worship of Vishnu however, by the Aryan conquerors of India or the original Dravidian inhabitants is not definitely known. In the ancient Vedas, the body of literature known as the Veda, and sacred literature of the Aryan conquerors, Vishnu is ranked among the lesser gods and is usually (falsely) associated with the major Vedic god Indra who in the epics and Puranas fights against dragons and demonic forces.
Later Vishnu was replaced in the same way by his avatar Krishna.