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.
A Blue moon is attributed to an extra full moon in the season, so the fourth Full moon of a season is a Blue moon since there are usually only three. There are 12.37 Full moons in a year, so a Blue moon must occur on average every 2.7 years.