Wait a minute. I'm getting confused here. Vulkan is not a full new FLTK platform.

It replaces what EGL does for the Wayland platform and runs inside a Wayland client.

Thus, Wayland will call wl_keyboard_key() at each keystroke. That function will compute

Fl::e_text and send FL_KEYBOARD events to your widget. It's your widget's job to draw 

the utf8 text received in Fl::event_text() adequately.

So, yes, all your widget has to do is follow what the documentation of functions

Fl::compose(), Fl::compose_reset(), fl_set_spot() and fl_reset_spot() say.