Like, notice how there's four women in this book, essentially: the two Sternwood sisters, Agnes, and Mona. They break down into pairs, naturally: the sisters of course, leaving the unrelated Agnes and Mona in the other corner. But Agnes and Mona make great sisters anyway, despite not being related; both are gangsters' molls, both spend time hidden away by their latest sponsor, and both end up on the run.
And there's a potential physical correspondence, too. Agnes, when we first meet her, is described as an "ash blonde" (p. 23 of the Vintage edition), while when we finally meet Mona she's a platinum blonde, although we learn this is a wig. They're both associated with the color green - Agnes wears black but we're reminded often of her green eyes, while Mona's wearing green when we meet her. Silver and green: they're money. No surprise that Marlowe thinks of Mona as "Silver-Wig."
So these two fugitive femmes are a fine match for the Sternwood sisters, a fine contrast, but in a more important way they're all four the same. Look at how the book ends:
On the way downtown I stopped at a bar and had a couple of double Scotches. They didn't do me any good. All they did was make me think of Silver-Wig, and I never saw her again.In fact, he's lost all four of the women by this point. Agnes has fled, after the successive murders of her two small-time crook boyfriends. Mona's in the wind. And Marlowe himself has demanded that Carmen be sent away, with a harshness and knowledge that suggests Vivian won't be in his life anymore either.
Of all four, it's Vivian we expect him to miss, and I don't think that's just retroactive because we can't escape Bogart and Bacall's chemistry; Vivian is the only one of the four whose intelligence, courage, and spirit match Marlowe's own. She's the one we naturally expect him to be attracted to. But it's Silver-Wig he misses. Why?
She freed him. She saved his life. She's the only one of the four who wasn't in some way trying to hurt him, and he knows it. That his last thoughts, in this book, are of her, not of Vivian anyway, tells us a lot, maybe everything, about this character. He plays with being self-destructive, with his whiskey and his cigarettes, but in the end he isn't: he's not hung up on the femme fatale.