Shoulder, knee and arm problems have limited the No 8 to just 249 minutes of rugby over the past year, with a recurring hamstring strain halting his latest comeback.
His return is penciled in for Saracens' play-off semi-final against Wasps — with the powerhouse forward hoping to shed half a stone and make a late claim for England's summer tour.
"I was doing well until about four weeks ago," he says. "I tweaked my hammy and mentally I was a bit broken by it. Instead of turning to partying, I turned to eating cake. My missus loves baking so it was lemon sponge cake, chocolate slice, the full range. I still need to find my best coping strategy.
"My best weight is around 130kg. I'm probably around 135kg at the moment. I'd love to go on tour with England in June but I just need to lose weight. I know I'm fat. I've spoken to Eddie Jones. He'll probably message me about keeping off the cakes after he reads this!"
Vunipola has not played for England since the final game of last year's Six Nations — although he hopes to be named in Jones's squad on Thursday. His absence was a key factor behind England's Six Nations implosion and, following that, it is clear that he is indispensable to their World Cup hopes.
"It was tough to watch, more because of what people were saying about us," says Vunipola.
"People were saying stuff that wasn't nice to take and it hurts your ego. My mum gave me a book about how to control the tongue, because it's full of evil. I shout at the screen, kick chairs, throw stuff. My fiancée gets a bit scared!
"People will say, "Oh, you need to win all the time', but unless you're Floyd Mayweather and take two years off in between games, then it's hard to win all the time. It's hard to always pretend the next game is always the biggest game. That's the truth. Maybe it's not a bad thing to go through a run like that, because it motivates you in future."
On the darkest days of his injury rehabilitation, Vunipola turned to the Methodist Church in St Albans for motivation. The 25-year-old has been one of the leading voices against player burn-out, threatening strike action after claims that players could not handle the demands of club and country.
"I don't think I could deal with any more long-term injuries mentally," he says. "People say, 'Oh yeah, you get five weeks off' but I don't think that's enough. Some boys will spend half of that recovering from surgery.
"I'm 25 now, I want to play til I'm 34 maybe. I'm contracted to Saracens for another year or so and I want to stay here after the World Cup. Maybe I'll stay at Sarries my whole career, but you want to experience life as well.
"My fiancée wants to go back to Australia at some point. I'm not sure how much longer my mum and dad will hang around here. It's too cold for them. I'm not bothered about the money, it's about the life experiences. Not straight after the World Cup but maybe later on, God willing."
Vunipola could be a candidate for the 2023 World Cup if his body holds out to the demands of modern rugby.
"It's disappointing to be out of Europe but hopefully we can fly into the semis," he says.
"Because we've always been in knockout stages, we've accumulated an extra 22 games in five seasons and that's probably added to our injury problems.
"If it wasn't for my faith, then I wouldn't be where I am now. For me, it's my best form of armour. It gives me something to fall back on. I haven't been as diligent with it as I should have been in the past, but religion has helped me with these injuries.
"I think I'd have lost patience if it wasn't for my faith. I started going back to church, consistently, with my fiancée and cousins."
Vunipola will be back at church this weekend. Then he will turn his focus towards his next comeback, with only days to prepare, and English rugby will once again hold its breath.
"These injuries have taught me a lesson," he says. "I took things for granted before. My head always gets too big. I try to pretend that it doesn't, but it does and it's something I'll always battle against — that and my weight!
"God has a bigger plan for me and he's just preparing me the best way he can. Hopefully I'm ready for it now."
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