The 22-year-old prop was a fringe player with potential at Saracens at the beginning of the season and was still waiting for his first Premiership start.
But having been put on a strict diet and rigorous training program Vunipola has propelled himself to a Saracens regular, an England super-sub and Lions squad member.
"It's been a massive attitude change for me personally," said the 20st 6lb prop.
"It's been a sudden rise. If someone had said three years ago a Lions place was possible I would probably have paid them money"
"When I first started at Sarries I had a bad habit of eating out and snacking here and there.
"For me it's been that attitude change, knowing that every bit counts. It's been tough for me to get used to, but after a year of it, it's paying dividends now.
"As a player it's all on your own back. You can only go as far as you want. For me it was a massive thing to push myself, knowing that I had to get myself fit.
"It's important to keep my feet on the ground and not got ahead of myself and not be too big-headed.
"To get the Lions call-up was a massive honour. To represent the Lions as a player is the highest honour. I'm very pleased and happy that I'm there with two of my team-mates [Matt Stevens and Owen Farrell]."
Vunipola was born in Wellington, New Zealand but moved to Pontypool in South Wales when his father Fe'ao, a former Tongan international, joined the famous club.
He began his club career at Bristol, having been educated at Millfield School. He only joined Saracens in 2011 but has enjoyed a breakthrough year.
He featured throughout England's RBS 6 Nations campaign and will face fellow Lion Dylan Hartley in Saracens' Premiership play-off with Northampton Saints in Sunday.
"It's all happened very quickly," added Vunipola.
"This time three years ago I was probably watching everyone else play. I would have watched Six Nations, then the Heineken Cup and Saracens' play-off semi-final.
"It's been a sudden rise. If someone had said a Lions place was possible I would probably have paid them money.
"At that point I had no idea that everything would happen so quickly. It's just been enjoyable playing my rugby for Saracens."