How to explain the westward shift of vernal equinox? There seems to be no nutation, but precession and pole shift do.

It is an illusion to March westward at the vernal equinox! The vernal equinox is a fixed point in the earth's orbit. Newton's solution to the precession of the earth's axis is a firm denial of the westward shift of the vernal equinox! If Newton thought that vernal equinox was moving westward, Newton used the tropic year when the earth's revolution was less than 360 to solve the precession.