Is GLSL the standard shading language for Vulkan?
Vulkan does not contain a normative reference to any high-level language specification. The only requirement is the ability to accept shaders in the SPIR-V format. For the convenience of the developers, an extension to GLSL was defined to allow it to generate Vulkan-compatible SPIR-V.
There is no barrier for other methods of generating valid SPIR-V. The SDK includes tools that you can use to validate whether a SPIR-V program is legal for Vulkan and should be accepted by a Vulkan implementation.