Spanish Football Federation rules do not prevent Vinicius from dropping back down to the 'B' team to help their Segunda B play-off bid.
After arriving at Real Madrid last summer, Brazilian starlet Vinicius was added to the ranks of Los Blancos' reserve team, Castilla, to take his first steps in Spanish football - a course of action which attracted its fair share of criticism.
"The competition is undermined by [the presence of] particular players of a top standard," complained Cultural Leonesa general manager Felipe Llamazares, whose side compete alongside Castilla in Segunda División B, Spain's regionalised third tier.
Having scored four goals in just five Segunda B appearances, Vinicius was then promoted to first-team duty after Julen Lopetegui's sacking as Madrid coach saw Castilla boss Santiago Solari placed in charge of the seniors at the end of October.
Vinicius can switch between teams "without limitations"
Now, with Castilla about to go into the play-offs for promotion to Segunda División A, Vinicius could drop back down to give the 'B' team a hand - at least, Spanish Football Federation (RFEF) rules do not prevent the 18-year-old from doing so...
The RFEF permits players under 23 who are registered as reserve-squad members to switch between clubs' first and second teams "without limitations". Vinicius fulfils these criteria as he is still officially listed as a Castilla player.
Brazil Copa América call-up would clash with play-offs
However, although the regulations offer no obstacle towards Vinicius turning out for Castilla after a six-month hiatus - he last played for the side in a 1-1 draw with Celta B on 21 October - international commitments may render the rule book moot.
Brazil coach Tite is due to name his squad for this summer's Copa América later this week, and the uncapped winger appears well placed to be included. This would definitively rule him out of the play-offs, as they would clash with the Seleçao's tournament preparations.