Learn Italian: Quel ragazzo estroverso è proprio vanitoso!

You would say Sì, lo è! In English, we often just leave the sentence hanging ("Yes, he is"), but Italian requires the invariable pronoun lo to stand in for the adjective phrase (proprio vanitoso) that you are replacing.

Why does this sentence use Quel instead of Quello?

When used as an adjective (meaning it directly precedes a noun), quello follows the same patterns as the definite article. Because ragazzo takes the article il, quello shortens to quel. You would only use quello before nouns that take the article lo (like quello specchio).

Could I use molto instead of proprio here?

You could, but they have slightly different flavors. While molto simply means "very" and indicates a high degree, proprio translates closer to "really," "truly," or "exactly." Here, it adds an emotional emphasis or confirms an observation, sounding a bit like "He sure is vain!"

Can vanitoso also mean "vain" in the sense of a "vain attempt"?