The vernal equinox is just another point in the sky (where the sun crosses the celestial equator on it's way into the northern sky), so it rises and sets just like any other point in the sky. When it crosses the upper meridian, it is directly south and up in the sky where you can see it. (When it crosses the meridian on the lower side, it is below the earth to the north.) The zenith is a point on the meridian directly above you. The meridian itself is just a circle running 360 degrees above and below you, from north to south.

When the sun crosses the meridian during the day, it is also directly south. During the next 24 hours, the earth moves in its orbit, so the earth has to rotate a little longer before the sun is again directly south (3 min, 56 sec. of time), so the solar day is longer than the sidereal day.

Hope this helps.