What will happen if we run our steam turbine in low vaccum?

The Steam consumption will be incresed.

In a condensing type turbine it would be a good efficiency or a relative good output as compared to steam input.  The larger the difference in enthalphy from the inlet of the turbine up to its last stage expansion is the higher the efficiency.  Consider this, vaccum pulls, and positive energy pushes.  So at the start of steam expansion at the first stage the energy is too high that it pushes the blades accordingly as to its energy equivalent and as it go down to its last stage the energy declines and if at some point in the condenser if the energy is low or negative or in a vaccum state it pulls the blades, so as a result it creates a smooth turbine operation thus it would create a greater efficiency.