Fe'ao, a former Tongan international, fully understood the extent of Billy's destructive qualities when a family presence at Burnham-On-Sea Rugby Club saw him taken out of his comfort zone.
Fear was replaced by self-belief when the 20st giant, who is expected to start England's opening QBE International against Australia at number eight, effortlessly floored an attacker.
"Basically my dad wanted to see if I was scared or not. I was so scared when I played my first men's game at Burnham, but I didn't want to show my dad I was scared," Vunipola said.
"I just thought 'if I'm good enough to be here I'm good enough to play'. That's something my dad drills into us - it doesn't matter how old people are.
"I played 12, outside my uncle, and in my first game we were running up to tackle someone. All I could hear was my uncle shouting 'hit him, hit him!'
"But I was too scared to tackle him so I just let him run into me. I didn't see if he was big or not but I could tell it was a good hit because my uncle was tapping me on the head.
"Ever since then I've had the confidence that I can do anything - I don't care who I play."
Harder tests followed rapidly as Vunipola left Bristol after winning a scholarship at Harrow School and from there he joined Wasps' academy where he was immediately thrown in at the deep end.
"When I rocked up at Wasps for my first pre-season I thought I was just going to watch training and see how everything was, but they said, 'we're doing a tackle drill, put a suit on'," the 20-year-old said.
"I said 'are you looking at me?!' I don't think anyone there knew how old I was. I think they all thought I was older than I was.
"But I had so much confidence from playing that game for Burnham-on-Sea. I remember being told that day at Wasps that some of the boys will test you, but that's just the way it is.
"Then lo and behold, the first carrier went step, step, step, straight into me. I tried to make the hit as best I could. Ever since then it's been fine."
An explosive start to the season at Saracens is likely to be rewarded with selection ahead of Ben Morgan against Australia on Saturday in what would be his first start for England after winning two caps as a replacement during the summer tour to Argentina.
The Sydney-born 6ft 2ins Vunipola joined older brother Mako at Allianz Park over the summer and has profited from a strength and conditioning programme that has seen him shed fat and pack on lean muscle.
A taste of the demands made of a professional rugby player came when growing up Griffithstown, Pontypool, having moved to Wales.
"Our house was on the corner of a block with two hills at each end. We'd run around the block and dad would time us," he said.
"We'd run up the hill, run across, run down the other hill and then sprint across. To the left of our house was an even bigger hill and we'd finish off up there.
"I'm not sure what it was called but it was massive. I was six or seven!
"When we first did it my mum and dad got in a few fights over it. It sounds pretty weird that you're making six-year-olds run!
"It felt pointless at the time but it helped me understand what hard work was.
"I was a bit overweight because when we came over (to the UK) I was probably a bit spoilt.
"My mum was always buying us McDonald's and we just thought it was normal.
"My uncles used to tell funny stories about me. I used to walk around in games until someone mentioned I might get £10 after the game. Apparently I'd run off and score four tries.
"But we don't have the gene to burn it off while doing nothing, so I always had to work harder than skinny boys."
- Sports & Recreation