Celebrity-beloved brands are having a moment this Amazon Prime Day. First, Gap slashed the $89.95 price tag on Hailey Bieber's favorite jeans in half. Then, Bella Hadid's Coach bag became one of Prime Day's biggest sales (if only for 24 hours). Now, the label behind Hadid's signature ballet sneakers made them more wallet-friendly than ever.Fashion girls got to know Vivaia in the summer of 2023, when the brand's best-selling ballet flats earned S…