The gravitational energy released as the sphere expands is equal to the rest-energy of the shell only if the sphere is larger than the Hubble sphere. Hence, in Newtonain terms, one finds that the gravitational binding energy released as the sphere expands is "converted" into new vacuum energy.
The numbers used here are useful when considering the case of inflation (which needs a tiny seed region to start), but the same reasoning applies to dark energy.
Figure: Whittle's (upcoming) undergraduate text.