Vulkan was designed to tackle several issues OpenGL had. For example, OpenGL does not do multithreading, while Vulkan is able to, and OpenGL has a single global state, while with Vulkan you're able to create multiple different command buffers to hold the state in. In general Vulkan is also a lot faster at rendering, as it's closer to modern GPUs in design.

While Vulkan has been slowly replacing OpenGL, it's also more complex than OpenGL is. This means that OpenGL is still great for many simple use-cases, as you won't be overwhelmed by the complexity that Vulkan can bring along with it.