The following contains spoilers for Black Widow.
Originally scheduled to hit theaters in May of 2020, Black Widow saw two different delays pushing the debut back. Black Widow finally made its way to the big screen (and Disney Plus) in July 2021. It's the title character's first solo movie, set during the events of Captain America: Civil War. Fans have known Black Widow since her Iron Man 2 debut. This missing chapter of the character's life, however, fleshes out a bit more of her backstory.
It also serves to introduce Yelena Belova to the Marvel Cinematic Universe. Yelena, raised in the Red Room like Natasha Romanoff, is Natasha's little sister with a love for the song "American Pie" and one very important piece of clothing. MCU fans were quick to spot her vest in trailers for the movie. It is the same one Natasha wears during the events of Avengers: Infinity War. Fans were intrigued about the implication of Natasha having Yelena's vest.
The Vest Demonstrates Yelena's Bond With Natasha
When Natasha and Yelena first reunite in the movie, they aren't exactly on friendly terms. They haven't seen one another since (at the very least) Natasha became a SHIELD agent. Natasha thought her defection to SHIELD brought down the Red Room. She discovers the opposite: Yelena has been one of their top agents ever since.
While Yelena has always considered Natasha to be her sister, Natasha is more hesitant to voice that. She has been trying to move on from the Red Room and atone for her actions ever since Clint Barton helped her escape in Budapest. Yelena is a reminder of the childhood she was robbed of and all of the bad things she's done. As Black Widow continues, however, and the two unite their spy "family" to take down the Red Room once and for all, Natasha freely admits that Yelena is her little sister.
That familial bond drives the movie even more so than Natasha's attempt at redemption. Yelena giving her the vest that she spends entire scenes talking up is a nod to their bond as sisters solidified again.
Yelena Finally Gets To Follow Her Own Path
When Yelena first begins talking to Natasha about her clothing, it's while the two of them drive to begin enacting their plan to find the Red Room. She extols the virtues of the vest. Yelena also makes a very important point about it: the vest is the first thing she chooses for herself.
It's the first piece of clothing she's been able to buy. As the audience sees, the Widows of the Red Room all have a uniform for missions and training. General Dreykov controls every aspect of their lives from their reproductive options to the clothing on their backs. Melina's demonstration of the chemical alterations to their brains reveals he even controls whether or not they're allowed to breathe.
The vest represents the first true choice Yelena has in her life. Sure, Yelena takes the serum to free the Widows from Oksana, but that's not a choice. Yelena views that as a responsibility. She even tries to give that to the Avengers. Other than declaring "American Pie" her song as a six-year-old, the vest marks the first time Yelena has something she views as completely hers. She gets a choice in it. Yelena chooses to give it to Natasha because she knows by the end of the movie that it's not the only choice she'll be able to make.
The Black Widows' Vest Has Pockets
Yelena's biggest selling point about her vest is the vast number of pockets it has. When she talks it up to Natasha, that's a point she makes repeatedly. Yelena loves that the vest has so many pockets to store so many things; though she does admit to still making modifications to the vest.
This particular point about Yelena's vest proves that the team behind Black Widow has its finger on the pulse of women in a modern society. If there's one thing women lament about fashion, it's the lack of function - specifically, the lack of pockets. When women's clothing is compared to men's clothing, the pockets are often smaller, shallower, or non existent. Things like keys or phones often don't even fit in them.
Someone like Yelena, who carries a variety of small weapons on her person wouldn't love the Black Widow catsuit and its lack of pockets. Sure, the suit has widow's bites in the wrist gauntlets and a few supplies in its belt, but that's it. The Iron Man 2 version of Natasha's suit has even less storage capabilities. Yelena's vest speaks to a desire from women everywhere to be able to store their belongings, even those in her line of work.