The difference between Giant and Vast

When used as nouns, giant means a mythical human of very great size, whereas vast means a vast space.

When used as adjectives, giant means very large, whereas vast means very large or wide (literally or figuratively).

check bellow for the other definitions of Giant and Vast

Giant as a noun:

A mythical human of very great size.
Giant as a noun (mythology):

Specifically, any of the gigantes, the race of giants in the Greek mythology.
Giant as a noun:

A very tall person.
Giant as a noun:

A tall species of a particular animal or plant.
Giant as a noun (astronomy):

A star that is considerably more luminous than a main sequence star of the same temperature (e.g. red giant, blue giant).
Giant as a noun (computing):

An Ethernet packet that exceeds the medium's maximum packet size of 1,518 bytes.
Giant as a noun:

A very large organisation.

Examples:

"The retail giant is set to acquire two more struggling high-street chains."
Giant as a noun:

A person of extraordinary strength or powers, bodily or intellectual.
Giant as a noun:

jotun

Vast as an adjective:

Very large or wide (literally or figuratively).

Examples:

"The Sahara desert is vast."

"There is a vast difference between them."
Vast as an adjective:

Very great in size, amount, degree, intensity, or especially extent.
Vast as an adjective (obsolete):

Waste; desert; desolate; lonely.