The Vernal Equinox is in which constellation?
The Vernal Equinox is currently in the constellation Pisces.
The Vernal Equinox was originally called the First Point of Ares in 130BC by Hipparchus. In those days the Sun crossed the celestial equator in the constellation of Ares.
Due to the precession of the equinoxes, this position has rotated such that the Sun now crosses the celestial equator in the constellation of Pisces.
It is the orbiting Earth that sees opposite constellations (one for night and the opposite for daytime, in a day of 24 hours. On vernal equinox (20 Mar) night, Earth might turn from Pisces to Aries
There is no precise separator-plane marked for transition, from one constellation to another. Only fringe stars can decide on whether it is Pisces or Aries. The day-hemisphere of Earth sees the antipodal Virgo to Libra transition, on this day. .