say it: VAST-lee
vastly is an adverb meaning "to an exceedingly great extent or degree". It carries one recorded sense.
to an exceedingly great extent or degree
“He had vastly overestimated his resources”
“was immensely more important to the project as a scientist than as an administrator”
Definitions, synonyms and semantic relations for vastly come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.