The Switch can use Vulkan but it's unusual in offering a wide range of APIs, from OpenGL and Vulkan (the implementations likely derived from Nvidia's existing PC driver) or a custom low-level API tailored to the hardware called NVN. From what I gather from the emulation scene, the majority of Switch titles with non-trivial performance requirements use NVN. Even idTech, which famously uses Vulkan on PC, uses NVN instead for its Switch ports.