Language is like clothing.
Those with no taste - but enough money - will dress in gaudy ways to show off their wealth. The clothing is merely a vector for this purpose. They won’t use a piece of jewelry only if it contributes to the ensemble. Oh, no. They’ll drape themselves with gold chains and festoon their fingers with chunky diamond rings. Brand names will litter their clothing. The composition will lack intelligibility, cohesiveness, and proportion. It will be ugly.
By analogy, those with no taste - but enough vocabulary - will use words in flashy ways to show off their knowledge. Language is merely a vector for this purpose. They won’t use a word only if it contributes to the prose. Oh, no. They’ll drape their phrases with unnecessarily unusual terms and festoon their sentences with clumsy grammar. Obfuscation, rather than clarity, will define their writing. The composition will lack intelligibility, cohesiveness, and proportion. It will be ugly.
As you can see, the first difference is one of purpose: the vulgarian aims for the wrong thing.
You might also say that the vulgarian also lacks a kind of temperance in speech.