The loosehead prop had been turning out for Bedford Blues in the Championship before Sarries came calling.
The original intention was reportedly to use Vunipola on tour as an impact player from the bench but as the injury curse took down Cian Healy, Gethin Jenkins and, briefly, Alex Corbisiero, Vunipola was parachuted into the starting line-up for the second Test in Melbourne.
And Vunipola stayed on the pitch for the full 80 minutes, winning penalties for his side by the end of the match and making more tackles than any other player on the pitch.
Corbisiero's calf injury subsided in time for the third Test but Vunipola again featured off the bench, as he had in the first Test.
That meant Vunipola flew home a winning Lion with three Test caps, not bad for a 22-year-old who had not started a Premiership game for Saracens before last season.
"Representing the Lions was undoubtedly the highlight of my career," he said.
"A year ago I just wanted to get a chance with Saracens - it all happened so quickly.
"I made friendships that will last for life, and to be around so many top professionals - Brian O'Driscoll, Paul O'Connell, Adam Jones - I learned so much, in terms of attitude, professionalism and responsibility.
"There was a lot to take in and I was a little overawed at times, but I really enjoyed it all.
"To be named in a starting team is an honour in itself, but to represent the Lions is the biggest thing,"