The 24-year old Chilean forward has penned a season-long loan deal with the R's after spending the second half of last season with Spanish club Valencia.
"I've spoken to Mauricio a lot and he said it's a great, relaxed environment to work in, with a fantastic squad of players," he told the club's official website.
"The Premier League is the best in the world, the most exciting in the world, and I am looking forward to competing in what is a super-competitive division."
Vargas initially joined Napoli from Universidad de Chile in 2012 but has scored just three goals in 28 appearances for the Italian side. He has spent time on loan with Gremio and Valencia since his arrival in Naples.
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