Is the vernal equinox always in zenith somewhere on the equator? I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses equator from south to north makes sense, as long as the vernal equinox is always on the equator. And I suppose it usually is. But the Earth's axis precesses--how can the RAAN make any sense, if the vernal equinox is no longer directly above the Earth's equator?
The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator.

Given the various ways that objects in orbit over Earth are perturbed, any set of keplerian orbital parameters that describe the orbit of any object over the Earth (including the Moon!) will need updating long before the drift of the Vernal Equinox along the celestial equator becomes significant.