What did hydrothermal vents do during Earths first years?

Answer 1

In the earlier of Earth's years; the Hadean from memory; there was no water and hence no hydro- anything. This was the first 700 million years or so. (Between 4.5 and 3.8 billion years ago.)

But some very old banded iron deposits come from this early, indicating that these were precipitated by changing chemical conditions.

Another few hundred million years later, the blue-green algae appeared, and having invented photosynthesis with its attendant release of oxygen, created the large banded iron deposits. And some soft-bodied fossils, such as the Ediacaran.

Answer 2

Hydrothermal vents likely played a crucial role in Earth's early years by releasing minerals and nutrients into the oceans, which supported the development of early life forms through chemosynthesis. They may have also contributed to the cycling of important elements in the early Earth's environment.