[1] Vitalii Mandzyn made his international debut at the 2020 Youth Olympic Games in Lausanne, where he finished fifth in the sprint and missed out on a medal by less than 20 seconds.

In the winter of 2021/22, he competed in the IBU Junior Cup and achieved a podium finish with the relay team.

Mandzyn won his first medal in the IBU-level in the junior competitions at the 2020 Summer Biathlon World Championships, where he was only beaten by the Czechs Tomáš Mikyska and Jonáš Mareček in the pursuit.

Vitalii finished his first relay alongside Artem Pryma, Dmytro Pidruchnyi and Anton Dudchenko in 13th place.

[3][4] At the World Junior Championships in Otepää, he won a silver medal in the Relay together with Stepan Kinash, Bohdan Borkovskyi and Serhii Suprun.