I don’t think it is Vulkan. At least not in Totem.
I’ve tried some of the GDK_DISABLE values and even a list of all of the values with Totem and still get a green video.
For Showtime, if I run GDK_DISABLE=vulkan,gl showtime … or GDK_DISABLE=vulkan,egl showtime … then I avoid the crash, I see Gsk-WARNING: Failed to realize renderer 'GskGLRenderer' for surface 'GdkWaylandToplevel' in the output and the video renders correctly. If I could set that just for Showtime then that could be workable, but I don’t want to disable Vulkan and GL globally!