A

Step 1: Understand what a vibration magnetometer is. It is an instrument used to measure the intensity of a magnetic field.
Step 2: The magnetometer should measure the horizontal component of the magnetic field accurately, which is only possible when it is aligned correctly.
Step 3: The magnetic meridian is the direction towards the magnetic poles, where the magnetic field lines are vertical.
Step 4: By setting the vibration magnetometer in the magnetic meridian, it ensures that the aligning process accurately reflects the local magnetic field conditions.
Therefore, the correct positioning is Option A: In magnetic meridian.