First of all, catch a Hitmonchan and raise it to lvl.26, as that is the level at which it learns Vacuum Wave. (all Hitmonchans are male)
Then, sling it in the Daycare with a female Lucario (this will be a problem, as most Lucarios are male, so you'll have to search hard for a female one) and their offspring will be cute baby riolus with Vacuum wave on them.
Then just evolve your Riolu into a Lucario by max happiness then you got your Vacuum Wave Lucario!
Source: I did it myself!