There is a lot that SFML is doing behind the scenes for you with sf::RenderWindow. I don't imagine we should expect a really high FPS unless the devs designed it to use Vulkan.
There's a lot more work involved in using the Vulkan API. The Vulkan implementation of SFML would probably be much larger since it expects us to do more of the heavy work. It might be faster, but it will be more complex. I for one very much like the simplicity of SFML's wrapper. Vulkan does less error checking than OpenGL, so it would be easier to cause a crash. One can use validation layers, but they can have an impact on performance. Would SFML force the use of validation layers internally or make the programmer use them manually?