  Aims. The very weak magnetic field detected at the surface of Vega hints at a
widespread population of weakly magnetic stars of A and B spectral types. We
contribute here to gather more clues about the origin of this magnetism by
investigating the long-term stability of the field geometry of this
prototypical star. Methods. We use spectropolarimetric data collected as part
of a long-term campaign, with more than 2,000 observations spread between 2008
and 2018. Using various sub-sets extracted from the whole time series, we
reconstruct several maps of the large-scale surface magnetic field. Results. We
confirm that the polarimetric signal is modulated according to a $\sim 0.68$ d
period, which we interpret as the stellar rotation period. The surface magnetic
field is organized in a complex geometry. We confirm the existence of a very
localized, polar magnetic spot previously reported for Vega, with a radial
field strength of about -5 G. We show that the surface of the star is also
covered by a dipole, with a polar strength close to 9 G and a dipole obliquity
close to $90^\circ$. Both magnetic structures are remarkably stable over one
decade. The available data suggest that smaller-scale magnetic spots may not be
limited to the polar region, although the poor reliability of their
reconstruction does not allow us to firmly conclude about their temporal
evolution.
