Vacuum or Boost is the result of high pressure forcing itself into low pressure environment

High (positive) pressure naturally forces itself into a low (negative) pressure environment in order to equalise pressure. Therefore, a pressure difference must be initially created for air to move from a high pressure to a low pressure environment in order to produce vacuum or boost.

For example,
To produce vacuum a lower (negative) pressure environment below atmospheric pressure must be initially produced inside the engine (ie: when a piston moves down the bore it reduces the pressure in the cylinder) while the higher atmospheric pressure outside the engine remains constant.

To produce boost a higher (positive) pressure environment above atmospheric pressure must be initially produced outside the engine (ie: when an air pump compresses the air it increases the atmospheric pressure) while the lower pressure inside the engine remains constant at a given rpm.