While Vinicius Junior was still on a development team contrast he would first come in to the first squad to make his debut with the “real” Real Madrid team. That would allow him to wear the number 28 even though it is required in the Spanish league that players wear a number a between 1 and 26.
Ronaldo had left Madrid for Turin thus new talent was required to blossom in order to fill the void.
Vinicius might not have been much of a difference maker during his first couple of seasons at the Bernabeu but step by step he began to rise to the occasion which would later move him quickly through the hierarchy of squad numbers from 28 to 25 to 20 and finally to number 7 which is current number at the time of writing late 2024.