  One of the most fundamental questions in cosmology is if dark energy is
related just to a constant or it is something more complex. In this work, we
call the attention to the fact that, under very general conditions, dark energy
can be identified with a cosmological constant. Indeed, this fact defines what
we call Vacuum Frame. In general, this frame does not coincide with the Jordan
or Einstein frame, defined by the invariant character of particle masses or the
Newton constant, respectively. We illustrate this question by the introduction
of a particular scalar-tensor model where the different hierarchies among these
energy scales are dynamically generated.
