Source : Webster's Revised Unabridged Dictionary (1913)

Vastly \Vast"ly\, adv.
   To a vast extent or degree; very greatly; immensely. --Jer.
   Taylor.

Source : WordNet®

vastly
     adv : 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" [syn: {immensely}]