“vast” is a relative term, a comparative term, indicating that something is large compared to something else, usually a more standard or typical measure. “very great in size, amount, degree, intensity, or especially in extent or range ” (Merriam-Webster’s on-line dictionary) so you could talk about the vast space between atomic nuclei, vast spaces in contemporary sports arenas and concert halls, vast spaces of the uninhabited deserts, vast spaces in the minds some of our politicians, or vast spaces between galaxies. All are very different in type and scale, but all are reasonable uses, in that all are very large spaces compared to the typical measure of space in each context.