According to Vaishnava belief, Vishnu is the primary creator of the universe.
This means that while other deities might play roles in the maintenance or specific aspects of the cosmos, Vishnu is understood as the one who creates the basic universal shell. He also provides all the necessary raw materials needed for its construction. Further, Vishnu places the living entities within the material world, allowing them to pursue their free will. This doesn't necessarily preclude other deities from having roles; it simply establishes Vishnu as the foundational creator.
In summary, in Vaishnavism, the answer to "Which god created the universe?" is Vishnu.
