In September 2020, Vuk was sent on a one-season loan to Istra.[6][7] He immediately scored in his first appearance for Istra, opening the score against HNK Gorica on 19 September 2020, as the match ended 2–2.[8] He finished the season as the top goalscorer for the club, scoring six goals in 30 total appearances, before returning to Rijeka.[9]
Having played for numerous national youth teams, Vuk made his debut for the Croatia under-21 team against Lithuania on 17 November 2020. He was in the starting lineup before being replaced by Luka Ivanušec at half-time in a 7–0 win.[12] He was selected to compete in the 2021 UEFA European Under-21 Championship, where he initially replaced an injured Luka Sučić, but had to withdraw from the tournament himself due to an injury.[13]