In monolayer TMDs (MX2), quasiparticle (or carrier) scattering by nonmagnetic disorder is highly sensitive to the spin-valley coupling which suppresses intervalley scattering when the spin-orbit splitting of the bands in the K,K' valleys is large. Here, we report on an additional symmetry-induced selection rule which relies on a three-fold rotational symmetry of the disorder site and strongly suppresses intervalley scattering irrespectively of the magnitude of the spin-orbit splitting of the bands. Based on atomistic first-principles calculations, we demonstrate the symmetry-induced vanishing of the intervalley matrix element for atomic monovacancies in 2D MoS2 and that the selection rule is highly sensitive to the position of the disorder site. Calculating the disorder correction to the electronic Green's function from the T-matrix, we show that our finding for the intervalley matrix element leaves a clear fingerprint in the Fourier transform of the real-space LDOS accessible in FT-STS. Altogether, our results suggest that long valley lifetimes are achievable even in disordered TMDs as long as chalcogenide vacancies are the dominant source of disorder.