His death was announced on Saturday by his team, Kyushu Electric Power Kyuden Voltex - known as Kyushu KV.
The Japanese second-division club said Vunilagi died on Friday after falling ill Monday and being hospitalised.
"After team practice on Monday ... Saimoni displayed symptoms consistent with severe heatstroke," the team said in a statement on Saturday.
"He was transported to a medical facility for treatment, but despite the best efforts of the medical team, he sadly passed away."
Kyushu KV is based in Fukuoka, a city in southwestern Japan. Temperatures there reportedly reached around 35 degrees Celsius on Monday.
"The players, staff, and all team associates are devastated by this sudden news," the team said.
Back rower Vunilagi joined Kyushu KV last month, having previously played in the Japanese top flight with Tokyo Sungoliath.
"Although Saimoni had only recently joined Kyushu KV, his generous spirit and powerful style of play had raised high hopes for his contributions in the upcoming season," Kyushu KV added.
"We humbly pray for the repose of his soul and extend our heartfelt condolences to his bereaved family."
The club supplied no other details surrounding the death. News reports said he was from Fiji.
A minute's silence was observed before the Japan v Australia Test match in Osaka. Vunilagi's former Suntory teammates Kanji Shimokawa and Naoto Saito both started for the hosts.
12°C