  The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers.
However, previous VLBI observations have been performed separately, which makes
it difficult to spatially trace the outward transfer of the material
consecutively. Here we present the astrometrically registered, simultaneous
maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VX
Sagittarii. The H2O masers detected above the dust-forming layers have an
asymmetric distribution. The multi-transition SiO masers are nearly circular
ring, suggesting spherically symmetric wind within a few stellar radii. These
results provide the clear evidence that the asymmetry in the outflow is
enhanced after the smaller molecular gas clump transform into the inhomogeneous
dust layers. The 129.3 GHz maser arises from the outermost region compared to
that of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser is
maximized around the optical maximum, suggesting that radiative pumping is
dominant.
