  We report Very Long Baseline Array (VLBA) observations of 43 GHz v=1, J=1-0
SiO masers in the circumstellar envelope of the M-type semi-regular variable
star VX Sgr at 3 epochs during 1999 April-May. These high-resolution VLBA
images reveal a persistent ringlike distribution of SiO masers with a projected
radius of ~3 stellar radii. The typical angular size of 0.5 mas for individual
maser feature was estimated from two-point correlation function analysis for
maser spots. We found that the apparent size scale of maser features was
distinctly smaller than that observed in the previous observations by comparing
their fractions of total power imaged. This change in the size scale of maser
emission may be related to stellar activity that caused a large SiO flare
during our observations. Our observations confirmed the asymmetric distribution
of maser emission, but the overall morphology has changed significantly with
the majority of masers clustering to the north-east of the star compared to
that lying to the south-west direction in 1992. By identifying 42 matched maser
features appearing in all the three epochs, we determined the contraction of an
SiO maser shell toward VX Sgr at a proper motion of -0.507 mas/yr,
corresponding to a velocity of about 4 km/s at a distance of 1.7 kpc to VX Sgr.
Such a velocity is on the order of the sound speed, and can be easily explained
by the gravitational infall of material from the circumstellar dust shell.
