From Shakespeare's Henry IV Part 1, Act III Scene 1:
GLENDOWER: I can call spirits from the vasty deep.
HOTSPUR: Why, so can I, or so can any man;
But will they come when you do call for them?
The phrase "vasty deep" is still somewhat used, and "vasty" appears in modern dictionaries, but only in reference to this one passage of Shakespeare. I'd guess that "vasty" might have been used instead of "vast" for reasons of rhythm, but was it a recognised word at the time, or was Shakespeare exercising some poetic license to invent a new variant of "vast"?