In vasp the symmetry is determined from the POSCAR file.
In vasp.4.4, the accuracy to which the positions must be correctly
specified in the POSCAR can be customized only during compile time
using the variable MY_TINY.
Per default MY_TINY is 10 
implying that the
positions must be correct to within around 7 digits.
If positions are not entered with the required accuracy VASP
will be unable to determine the symmetry group of the basis.