UEFA's control, ethics and disciplinary body (CEDB) imposed a six-match ban on Prestianni on April 24 after the player admitted using homophobic language towards Vinicius during a Champions League match on February 17.
An allegation he had racially abused Vinicius was found not proven.
Three of the six games were suspended for a period of two years, while one had already been served provisionally for the second leg between the teams.
Football's global governing body FIFA has now agreed to a request from UEFA to extend the ban worldwide.
The ban covers official international matches, so would include Argentina's opening two World Cup games but not any pre-tournament friendlies.
A FIFA spokesperson said: "The FIFA disciplinary committee has decided to extend the six-match ban imposed by UEFA on SL Benfica player Gianluca Prestianni to have worldwide effect.
"The sanction has been extended in accordance with article 70 of the FIFA Disciplinary Code."
Prestianni has only won one senior cap for Argentina to date, and the extension of the ban now makes his selection for the World Cup very unlikely. It is therefore most likely the ban will be served in next season's Champions League.
Prestianni covered his mouth while speaking to Vinicius after the Brazilian had scored for Real in the match in Lisbon.
In late February FIFA brought a proposal calling for players who cover their mouth when confronting an opponent to be shown a red card to the annual meeting of the International Football Association Board (IFAB), which sets football's laws.
That proposal received unanimous approval at a special IFAB meeting in Vancouver last week, which means competitions now have the discretion to apply the sanction if they wish to.
