Vanity and pride are different things, though the words are often used synonymously. A person may be proud without being vain. Pride relates more to our opinion of ourselves, vanity to what we would have others think of us.
Mary, the family moralist, draws a neat distinction that doubles as a thesis for the novel. The contrast slices through behavior we’ll see everywhere—from Darcy’s inward pride to characters chasing public admiration. Austen uses Mary’s bookish clarity to frame later events. The line helps us read people more carefully: are they guarding self-respect or courting applause? It’s a sober thought in a lively story. And it primes us to notice how both faults tangle hearts.