I don't know what a "vast" sentence is. Aside from that, you can find the answers to such questions in a dictionary. st/ [vast, vahst] adjective, -er, -est, noun This tells us that 'vast' is an adjective and a noun, that the comparative form is 'vaster', and that the superlative form is 'vastest' -- the '-er' and '-est' suffixes tell us that.
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