  We present simultaneous 0.65-2.5 microns medium resolution (3300 < R < 8100)
VLT/X-Shooter spectra of the young low-mass (19+/-5MJup) L-T transition object
VHS 1256-1257 b, a known spectroscopic analogue of HR8799d. The companion is a
prime target for the JWST Early Release Science (ERS) and one of the
highest-amplitude variable brown-dwarf known to date. We compare the spectrum
to the custom grids of cloudless ATMO models exploring different atmospheric
composition with the Bayesian inference tool ForMoSA. We also re-analyze
low-resolution HST/WFC3 1.10-1.67 microns spectra at minimum and maximum
variability to contextualize the X-Shooter data interpretation. The models
reproduce the slope and most molecular absorption from 1.10 to 2.48 microns
self-consistently but fail to provide a radius consistent with evolutionary
model predictions. They do not reproduce consistently the optical spectrum and
the depth of the K I doublets in the J-band. We derive Teff = 1380+/-54 K,
log(g) = 3.97+/-0.48 dex, [M/H] = 0.21+/-0.29, and C/O > 0.63. Our inversion of
the HST/WFC3 spectra suggests a relative change of 27+6-5 K of the
disk-integrated Teff correlated with the near-infrared brightness. Our data
anchor the characterization of that object in the near-infrared and could be
used jointly to the ERS mid-infrared data to provide the most detailed
characterization of an ultracool dwarf to date.
