Vacuum in the manifold drops as engine demand increases. If you use ported vacuum you always have a vacuum signal. If you want to make adjustments it can be done using the advance pot on the distributor when you use ported vacuum.
The decreasing or waning phases are when the moon shrinks from the full moon down to the new moon (darkness). As the moon wanes during the 3rd and 4th quarters, this is a good time to prune plants, as the water table is diminishing and so less sap will flow out of the cut ends.
Put garden plants in place after all danger of frost is gone, and bloom should begin in late spring in mild climates or early summer in cooler climates and last until late summer or fall. Plants go dormant in the winter and should not be overwatered at this time due to susceptibility to fungal diseases.