The win also means that Tonga has qualified ahead of Samoa for the Oceania 2 place in the 2019 Rugby World Cup in Japan.
As for Fiji, they finish top of the Oceania group and go into Pool D, along with Australia, Wales and Georgia, with a last spot still to be decided.
Vatubua ran off a pass from Nagusa to score in the 52nd minute and Ratuniyarawa crashed over from close range six minutes from fulltime.
Fiji scored five tries to secure the bonus point win. Flyhalf Ben Volavola kicked six goals from six attempts, converting all of Fiji's five tries and adding a penalty from 55 metres.
"We were our own enemies in the first half", McKee said.
"The benefits of sport start with fitness and improved health, but the young people who take part in the programme will also learn life skills like commitment, how to work together while making friends and having a good time".
Samoa coach Alama Ieremia was inevitably disappointed with the result, which will see his side slip one place to 16th when the World Rugby Rankings update at 12 noon on Monday, their lowest position since the rankings were introduced in 2003. "We let Samoa into the game with a number of penalties we gave away in the first half".
Seniloli scored his second when winger Vereniki Goneva made a break down the right flank. Fiji deserved the victory and we congratulate them. It's why we play rugby.
"During this campaign, we've set ourselves a benchmark", he said. "We will regroup and I am sure the squad will do the country proud in the future".
If Samoa wins its repechage it might have an easier road in World Cup pool play, playing in Pool A with Ireland, Scotland, Japan and a European qualifier. The victor will face the Asia Rugby Championship 2018 victor for a place in repechage tournament.