In my communications with Unigine, they've said they have been exploring the use of Vulkan but haven't yet made any firm commitments as they continue exploring this newer, high-performance graphics API. Obviously wiring up Vulkan to their massive graphics/3D engine will take some time. But it hasn't been for lack of interest in Vulkan either: Superposition on Windows has just a Direct3D 11 renderer with no D3D12 support.
The good news though is Unigine Corp has now publicly confirmed that they expect to have Vulkan support later this year, "Vulkan support is in the roadmap for the engine this year."
That's certainly exciting to hear and hopefully they will release an update to Superposition with Vulkan renderer when available.