  We report a high-resolution photocurrent (PC) spectroscopy of a single
self-assembled InAs/GaAs quantum dot (QD) embedded in an n-i-Schottky device
with an applied vector magnetic field. The PC spectra of positively charged
exciton (X$^+$) and neutral exciton (X$^0$) are obtained by two-color resonant
excitation. With an applied magnetic field in Voigt geometry, the double
$\Lambda$ energy level structure of X$^+$ and the dark states of X$^0$ are
observed in PC spectra clearly. In Faraday geometry, the PC amplitude of X$^+$
decreases and then quenches with the increasing of the magnetic field, which
provides a new way to determine the relative sign of the electron and the hole
g-factors. With an applied vector magnetic field, the electron and the hole
g-factor tensors of X$^+$ and X$^0$ are obtained. The anisotropy of the hole
g-factors of both X$^+$ and X$^0$ is larger than that of the electron.
