WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 180, "The Assassin, Mugino," now streaming on Crunchyroll.
In the Boruto manga, Himawari's vase has been a crucial part of Kawaki's arc after Naruto adopted the at-risk teen in Konoha. We knew Himawari gave it to her mom, Hinata, for her birthday but assumed it was simply important due to Naruto often neglecting the family for his job as Hokage. However, in Episode 180 of the Boruto anime, we discover the backstory to this item and just why it's so special to her kids.
At the start of Episode 180, Himawari is painting the vase, focused on making sure it's as gorgeous as can be. She even shoos Boruto away when he tries to inquire about it, only to realize he forgot his mom's birthday. Seeing Hinata making this emotive gift shocks him into action and he contemplates buying flowers, only to have to spend all his money to clear Mugino's debt.
Later, he discovers more about Mugino's life as an assassin who rehabilitated in Konoha, all because he was saved by Hiruzen, the Third Hokage. When they're finished chatting, a young kid comes by and drops flowers for Mugino, who didn't lie as Boruto initially thought over the tab Mugino owed. Apparently, he really did donate his salary to the kid's sickly sister, thus confirming to Boruto he really is a kind shinobi. In exchange, Mugino gives Boruto the flowers as payment and he goes home and puts it in Hinata's vase. Now, it's the perfect gift because it represents not just Himawari's ambition and innocence, but Mugino's warmth, kindness and compassion.
To Boruto, as all fans can attest to, this is simply Hinata in a nutshell. She was devoted to helping all her friends in Konoha and loved Naruto from day one, selflessly protecting him in the field. And Boruto realizes Mugino, while he's more comedic, is like his mom. It's why he was upset Kawaki accidentally broke it when he came to their home in the manga, and why Himawari was so sad. To them, it was a mark of fixing a broken family, something which also applies to Mugino as he was a war orphan who looked up to Hiruzen, not just as a savior, but as a dad.
Sadly, Orochimaru killed Hiruzen during Naruto's first exam, before Mugino could officially accept asylum and come to Konoha to learn the way of the shinobi. These flowers really do have decades of meaning and sentiment to them. By putting them in the vase, it's all about legacy, love and how brave and caring Konoha's ninjas, despite their past reputation, could be. Thankfully, Kawaki would repair it as he knew it meant so much to the Uzumaki household and now we understand a bit more why this was the case.