  Context. VX Sgr is a cool, evolved, and luminous red star whose stellar
parameters are difficult to determine, which affects its classification. Aims.
We aim to spatially resolve the photospheric extent as well as the
circumstellar environment. Methods. We used interferometric observations
obtained with the MATISSE instrument in the L (3 to 4 {\mu}m), M (4.5 to 5
{\mu}m), and N (8 to 13 {\mu}m) bands. We reconstructed monochromatic images
using the MIRA software. We used 3D radiation-hydrodynamics (RHD) simulations
carried out with CO5BOLD and a uniform disc model to estimate the apparent
diameter and interpret the stellar surface structures. Moreover, we employed
the radiative transfer codes Optim3D and Radmc3D to compute the spectral energy
distribution for the L, M, and N bands, respectively. Results. MATISSE
observations unveil, for the first time, the morphology of VX Sgr across the L,
M, and N bands. The reconstructed images show a complex morphology with
brighter areas whose characteristics depend on the wavelength probed. We
measured the angular diameter as a function of the wavelength and showed that
the photospheric extent in the L and M bands depends on the opacity through the
atmosphere. In addition to this, we also concluded that the observed
photospheric inhomogeneities can be interpreted as convection-related surface
structures. The comparison in the N band yielded a qualitative agreement
between the N band spectrum and simple dust radiative transfer simulations.
However, it is not possible to firmly conclude on the interpretation of the
current data because of the difficulty in constraing the model parameters using
the limited accuracy of our absolute flux calibration. Conclusions. MATISSE
observations and the derived reconstructed images unveil the appearance of the
stellar surface and circumstellar environment across a very large spectral
domain for the first time.
