Explain the un-ionized form of vanillin

It is the un-ionized form of vanillin that has the typical odor.  In NaOH solution, the phenol group ofvanillin ionizes to its conjugate-base phenoxide ion; because vanillin is no longer present, and because ionic compounds like the conjugate-base phenoxide are not volatile, the odor disappears.  (A compound has to be volatile-that is, it must have a significant vapor pressure-to enter the gas phase and therefore reach the nostrils.)  Acidification of the solution brings about protonation of the phenoxide and regeneration of vanillin and, as, the characteristic odor.