After Eddie Jones picked his squad for the tournament, the England head coach said he needed players who could "cope with the different culture" and it would seem Vunipola, 26, fits the bill.

England claimed their second World Cup win against the USA on Thursday and play Argentina next on Saturday so Vunipola has used the time off in between wisely.

"I managed to dive into the cuisine a bit more," the Saracens forward told BBC Sport.

"I've had some more Kobe beef, I'm still not convinced it's better than Miyazaki beef.

"We went and explored Osaka - it was crazy busy, I've never seen anything like it. We got a bit lost and felt a bit out of place by being so big."

According to scrum coach Neal Hatley, all the players have "recharged" like Vunipola and the whole squad is fit and available for selection for the first time this tournament, including wing Jack Nowell and Vunipola's older brother Mako.

The prop has not played since England's World Cup warm-up match against Ireland in August and Hatley says his return creates a "difficult decision", with Joe Marler and Ellis Genge both performing well against Tonga and the USA.

But injury-prone Billy, who broke his arm three times last year, says his brother will relish a return to play because "the alternative is way worse".

"He's doing well and he's enjoying himself more because he knows he'll be training with the team rather than on his own," he explained.

"As rugby players, training on our own is not something we enjoy. If he trains with us then he won't take that for granted because he knows the alternative is rubbish."

'Playing stops me eating rubbish'

Eddie Jones said he wanted players who could "cope with the different culture" of Japan

Billy started in all four of England's warm-up matches and has done so in both of their pool games so far.

The 20st 9lb forward says he is enjoying getting so much game-time, especially because it stops him overindulging.

"It keeps me off eating rubbish," he joked. "I'm all for it. If I keep my weight down it gives me more confidence.

"I've had a few injuries the last few years so to have an opportunity to play week in, week out is amazing. It gives me more confidence every time I play."

When he was not out eating and seeing the sights, Vunipola also managed to appreciate a stellar weekend of rugby as a fan.

Japan kicked things off with a shock victory against Ireland that sent the home crowd into raptures, before Wales came out on top in a thrilling encounter with Australia.

Japan's win was one of three games Vunipola watched on Saturday along with Argentina v Tonga and South Africa v Namibia.

"I love watching, it's a great experience," he added. "Sometimes you take it for granted that you're playing in the World Cup but to watch other teams competing just makes you more excited.

"I thought the Japan game was awesome - that's probably the best crowd I've seen all tournament.

"They played unbelievably well and there's a lesson in that for us. We're pumped after these two games but you know Argentina are going to come in hot."

'Argentina are our biggest challenge'

Argentina will provide England's toughest test so far and Vunipola acknowledged it would be "a step up".

England are top of pool C after two bonus-point wins and the Pumas are second following a narrow defeat by France and victory over Tonga.

Argentina will bring a strong pack when the two sides meet at Tokyo Stadium and Vunipola hopes his side can cope with it.

"This is the kind of pressure that we've been asking for," he said.

"We've done a lot of talking over the last five to six weeks about how together we are but we haven't had a chance to show it. This is our first big opportunity and I hope we don't disappoint.

"It's probably the biggest challenge to us as a forward pack. We know that's coming and we need to turn up emotionally as a group."