The 26-year-old winger marked the occasion with a magnificent try as the Tongans secured their first victory of the tournament, beating the Japanese 31-18.
Vainikolo, who was born in Tofoa, Tonga, played with Northland, Otago and the Highlanders before joining Connacht in the summer.
He was part of Tonga's World Cup squad along with Soane Tonga'uiha (Northanpton Saints), Taufa'ao Filise (Cardiff Blues) and former Connacht centre Andrew Ma'ilei.
Commenting on his return to Connacht, Vainikolo said: "Playing for my home country was a huge honour and being part of the World Cup was an amazing experience.
"For some players it's a once in a lifetime achievement, so I'm delighted to have had the opportunity to test myself at the highest level.
"Being back in the Connacht squad is great, the weather isn't quite the same but it's good to be back involved with the team and I'm looking forward to getting stuck into the league now."