Capture (hook) VSync (or VerticalBlank) of VR HMD in Direct Mode

Hi! I am trying to catch (hook) the VSync calls for steamvr app (vrcompositor), I hook into WaitForVBlank and same *km (OGL) API functions, but they are never called in vrcompositor.

Does anyone knows how the VSync is operated in direct mode? (Everything is understood in extended mode for Vive Oculus).