  We give a brief review of the known effects of a dynamical vacuum
cosmological component, the dark energy, on the anisotropies of the cosmic
microwave background (CMB). We distinguish between a "classic" class of
observables, used so far to constrain the average of the dark energy abundance
in the redshift interval in which it is relevant for acceleration, and a
"modern" class, aiming at the measurement of its differential redshift
behavior. We show that the gravitationally lensed CMB belongs to the second
class, as it can give a measure of the dark energy abundance at the time of
equality with matter, occurring at about redshift 0.5. Indeed, the dark energy
abundance at that epoch influences directly the lensing strength, which is
injected at about the same time, if the source is the CMB. We illustrate this
effect focusing on the curl (BB) component of CMB polarization, which is
dominated by lensing on arcminute angular scales. An increasing dark energy
abundance at the time of equality with matter, parameterized by a rising first
order redshift derivative of its equation of state today, makes the BB power
dropping with respect to a pure LambdaCDM cosmology, keeping the other
cosmological parameters and primordial amplitude fixed. We briefly comment on
the forthcoming probes which might measure the lensing power on CMB.
