The question was immediately responded to by David Airlie who commented that Gallium3D is higher-level than Vulkan, so such a state tracker wouldn't really work out, at least in Gallium's current form. Dave mentioned though that there could be refactoring going on to share pieces of Gallium3D drivers into shared compiler code.
Jose Fonseca of VMware commented with the idea of a "Germanium" abstraction that would be a superset of Vulkan with a similar API and exposing additional state that was found in OpenGL but not with Vulkan. It would make things easier to implement OpenGL.
Jason Ekstrand of Intel who is one of the main developers of the Mesa-based Vulkan driver brought up a few ideas of implementing OpenGL on top of Vulkan, but not without some obstacles.
Long story short, in its current form Gallium3D wouldn't really work for Vulkan, but there are different possible paths forward in the future should any talented developers find themselves with extra time and in need of an interesting experimental project.