The perpetrators used a black figure wearing Vinicius' No. 20 shirt, tied a rope around its neck and hanged it from a bridge near Real Madrid's training ground on the morning of a derby against Atletico Madrid.
Authorities said three of the suspects were members of an ultra fan group for a Madrid football team, but did not specify which club.
Police used security camera footage to identify the suspects, Spanish media reported, but no action had been taken until now.
Vinicius targeted by racist abuse
The arrests were announced two days after spectators in Valencia hurled racist insults at Vinicius during a match, sparking international outcry including from Brazilian President Luiz Inacio Lula da Silva.
FC Valencia banned one spectator for life for insulting Vinicius during the game.
Real Madrid said the incident was a hate crime and lodged a criminal case with prosecutors.