Eddie Jones' men face Australia at Oita Stadium knowing their quest to lift the Webb Ellis Trophy will be in tatters along with the Twickenham future of their head coach if they slump to a first defeat by the Wallabies since 2015.
Jones has staked the success of his entire regime on England's performance at the World Cup, while it would be Michael Cheika's final match at the Australian helm if his underdogs were to lose the quarter-final.
Four years of build-up to Japan 2019 hurtles towards a win or bust clash – a point rammed home with brutal honesty by Vunipola.
"Wednesday is our biggest training day and I said as a remark just to check the boys: 'lads, this could be our last session'," Vunipola said.
"I had a few stares from the lads and they just all laughed it off, but I was like, 'I'm being serious. If we don't turn up, we're going home.' I said it as a joke but it shocked a few of the boys.
"It is always nice to remind yourself. You don't want to live in fairyland because you get to Sunday and it's done and that's when the excuses come up.
"But if you just take it head on....we have talked about it already and made sure we know what the consequences are. We also know that if we can keep playing the way we have but just that little bit better, we will be in a good place."
Billy is to be reunited in England's pack alongside older brother Mako for the first time since the Six Nations victory over France in February.
Mako will be making his maiden start since May 11 after a hamstring injury interrupted his year, preventing him from making his comeback until the bonus-point victory over Argentina.
"It definitely is a thing playing with Mako. Subconsciously it's not something we think about, but having my brother there gives me that time," Vunipola said.
"He obviously takes away tension because he is as much of a threat. We've got the Kyle Sincklers and the Jamie Georges, but Mako gives me space and timings in the next carry.
"He takes a lot of pressure off me because he's the older brother, so anything that comes towards the Vunipolas he usually takes the brunt of it and I'm always in the back just kicking back as younger brothers do.
"I enjoy having him around, he's a bit like my shield. That's what big brothers do. I thoroughly enjoy playing with him and I'm happy to see him back in the team.
"We have a funny relationship. Deep down we really love each other but he never says it to me. I always say it to him but he gets embarrassed and runs away!
"There is definitely that love and respect. We just do our own jobs. We don't have to hang around and tell each other every day or every other minute how much we care for each other, it's just there."
Billy Vunipola at his best is an unstoppable ball carrier, but by his own admission the Saracens number eight – the only player to start all 12 of England's games this year – has yet to hit full throttle in Japan.
"I'm not being given as much time and space as I was used to. In the warm-up games teams were watching and I became a big target against Tonga," he said.
"It was about trying to change where I turn up, whether it was off nine, 10 or 13. It's about doing whatever I can to help the team, whether that's more depth off nine or 10 or just trucking it instead of trying to use footwork.
"I went from using no footwork to using a lot of footwork and now I think I need to go back. Hopefully that'll create space for me to use footwork later on.
"At the moment I'm not doing any tough carries, doing the hard yards which I probably haven't been doing. I have been happy with where I'm at. My job is to help the team and I feel like I have done that over the last three weeks."