  The u band magnitude is vital for determining stellar parameters and
investigating specific astronomical objects. However, flux calibration in the u
band for stars in the Galactic disk presents significant challenges. In this
study, we introduce a comprehensive re-calibration of $u$-band photometric
magnitudes of the VPHAS+ Data Release 4 (DR4), employing the Stellar Colour
Regression (SCR) technique. By leveraging the expansive set of XP spectra and
$G_{\rm BP}$ photometry from Gaia Data Release 3 (DR3), as well as the
individual stellar extinction values provided by the literature, we have
obtained precise model magnitudes of nearly 3 million stars. Our analysis
identifies systematic magnitude offsets that exhibit a standard deviation of
0.063 mag across different observational visits, 0.022 mag between various
CCDs, and 0.009 mag within pixel bins. We have implemented precise corrections
for these observational visits, CCD chips, and pixel bins-dependent magnitude
offsets. These corrections have led to a reduction in the standard deviation
between the observed magnitudes and the model magnitudes from 0.088 mag to
0.065 mag, ensuring that the calibrated magnitudes are independent of stellar
magnitude, colour, and extinction. The enhanced precision of these magnitudes
substantially improves the quality of astrophysical research and offers
substantial potential for furthering our understanding of stellar astrophysics.
