The control of the exciton intervalley coherence renders transition metal dichalcogenides monolayers promising candidates for quantum information science. So far, generating intervalley coherence has the need for an external coherent field. Here, we theoretically demonstrate spontaneous generation (i.e., without any external field) of exciton intervalley coherence. We achieve this by manipulating the vacuum field in the vicinity of the monolayer with a designed polarization-dependent metasurface, inducing an anisotropic decay rate for in-plane excitonic dipoles. Harnessing quantum coherence and interference effects in two-dimensional materials may provide the route for novel quantum valleytronic devices.