  In this paper we study the evolution of the FRW universe filled with variable
modified Chaplygin gas (VMCG). We begin with a thermodynamical treatment of
VMCG described by the equation of state $P = A\rho-B\rho^{-\alpha}$, and obtain
its temperature as a function of redshift $z$. We show that the results are
consistent with similar works on other types of Chaplygin gas models. In
addition to deriving the exact expression of temperature of the fluid in terms
of the boundary conditions and redshift, we also used observational data to
determine the redshift at the epoch of transition from the decelerated to the
accelerated phase of expansion of the universe. The values of other relevant
parameters like the Hubble parameter, the equation-of-state parameter and the
speed of sound are obtained in terms of the redshift parameter, and these
values are compared with the results obtained from previous works on MCG and
other Chaplygin gas models for the various values of $n$ permitted by
thermodynamic stability. We assume the present value of temperature of the
microwave background radiation to be given by $ T_0 = 2.7 K $, and the
parameter $ A $ in the equation of state is taken as $ 1/3 $ since it
corresponds to the radiation-dominated phase of the universe. The value of the
parameter $\Omega_x$ has been assumed to be $0.7$ in our calculation. Since it
is known that the redshift of photon decoupling is $ z\simeq 1100 $, we used
this value to calculate the temperature of decoupling.
