Can someone answer a question for me?

Not at all. She didn't have anything to prove. She and her then husband and director Guy Ritchie, made this film because they liked the original, and wanted to make their (modern) version of it. I'm pretty sure they knew they weren't making a masterpiece, and they knew that it was silly at moments, but they didn't care, they had fun.
And the extremely low budget ($10,000,000!!) says that they didn't care about box office either.

The film is not as bad as people say by no means. Sometimes some films get butchered by critics and people just because. Sure, it's not as good as the original, and it's silly and campy at moments, but it's serious and sentimental at others, and the main idea of it is really interesting. How roles can change when two people get isolated. We're forced to play a role because we're surrounded by others, by society, and we're afraid that we'll get fired or sued or whatever, if we do something inappropriate. But when a master and a servant find themselves alone for a period of time, they're not master and servant anymore, they're just two people.

As for Madonna, as you know she plays a character who at the beginning is a spoiled rich bitch, and then, when she and the skipper desert at the island, she's being humiliated by him, so no, there's not much "glamour" for her in the film. In fact, I was really surprised of how she let herself seem so old. She was just 43 at that time (they shot the film in 2001), and she looked way older than she was during the "Confessions On a Dance Floor" period, in 2005, when she was 47, and even later. Actually, I really didn't feel she was Madonna as I was watching the film. It was difficult to "recognise" her. She really looked much older than 43, and generally it wasn't "her". I couldn't believe she was the same person from all those '80s (and '90s) video clips. She could have demanded to look fabulous, like all those narcissistic movie stars, but she probably didn't care.

So to answer your question, no vanity here by her. It's in her music that she shows vanity, refusing to grow up, and putting tons of make-up and photoshop, and making s--tty egocentric songs like "Bitch I'm Madonna" to desperately keep up with the You Tube generation!
"Swept Away" was a peculiar little parenthesis in her career. It was a different Madonna.