Phase rule for a pure substance in vapor-liquid region
Note that the phase rule is qualitative. It says nothing about the particular amounts. It simply enumerates the theoretical number of independent parameters needed to determine amounts and compositions for the given number of components and phases.
The definition of a system, like total amounts of components, does not count among degrees of freedom.
The number of degrees of freedom of a system is the number of independent parameters, that are needed for the given system to determine its state.
For the given container volume and the substance amount, a given temperature determines the equilibrium pressure and vice versa. So there is 1 degree of freedom.
Amounts of particular  phases are determined by the temperature as the  single degree of freedom.