If you are interested in good steady game performance then I'm afraid FireMonkey isn't the best choice. You see FireMonkey is primarily designed for application development. While you can stil develop games with it it FireMonkey isn't the best choice for this.

By the way Firemonkey and Vulkan are two completely different things.
Firemonkey is a set of libraries that allows development of graphic rich applications. In order to do this Firemonkey uses advanced graphical API-s for content rendering like DirectX or OpenGL instead of using GDI rendering as the VCL does.
Vulkan is one of those advanced graphical API-s that allow programs to communicate with graphics card efficiently similar to DirectX or OpenGL.

As for Idera/Embarcadero adding Vulkan support to FireMonkey I seriously doubt this would solve all performance issues FireMonkey has. The main reason for this is the way how FireMonkey is developed internally. If FireMonkey performance would be only affected by the graphical API the FireMonkey uses for rendering the it should be reaching similar performance to other DirectX or OpenGL based graphical engines but it does not.