His explosive style off the replacement's bench has seen him rewarded internationally by both England boss Stuart Lancaster and Lions coach Warren Gatland.
But it was a wake-up call last summer at his off-field behaviour that has seen him win an opportunity of a lifetime with the Lions this summer.
"It's been a massive attitude change for me personally," said Vunipola.
"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."