This is the 1971st most frequent Portuguese word.
vinícius
"Vinícius" is a Portuguese given name; it has no standalone definition beyond being a proper noun.
In this sentence, the word 'Vinícius' is used as a proper noun, which is a name referring to a specific individual.
Vinícius é meu melhor amigo.
Vinícius is my best friend.
Here, 'Vinícius' is once again a proper noun, and it is used in a possessive context to indicate ownership or association of the book.
O livro do Vinícius está sobre a mesa.
Vinícius' book is on the table.
This instance uses 'Vinícius' as a subject, emphasizing the action performed by the person named Vinícius.