Vastly is a synonym of extremely.
As adverbs the difference between extremely and vastly
is that extremely is to an extreme degree while vastly is greatly, in a vast manner.
vastly
English
Adverb
(
en adverb)
Greatly, in a vast manner.
*{{quote-book, year=1913, author=
, title=Lord Stranleigh Abroad
, chapter=1
citation
, passage=“I have visited my quarters, and find them very comfortable.
Synonyms
* exceedingly, extremely, highly, immensely, supremely, tremendously