Vunipola only made his England debut in the autumn having had to wait until September to earn his first Sarries league start but he now finds himself on the verge of history with Britain and Ireland's elite this weekend.
"It's unfortunate with Corbs' injury, but that's the nature of the game. To be named in a starting team is an honour in itself, but to represent the Lions is the biggest thing," said Vunipola, who qualifies to play for Wales, Tonga and New Zealand as well as England.
"It's happened really quickly for me. This time last year I was back home just relaxing. To be here now is beyond my wildest dreams.
"There's been a lot of work done behind the scenes and with the help of the staff back at Sarries and England. I've been really lucky with the chances I've been given and lucky to impress the coaches.
"It'sdefinitely the ultimate reward. As a player, it's the highest honour to represent the Lions, the best of the four countries, the elite. To be starting for them on Saturday is the highest honour and I just feel really privileged.
"The whole squad knows what this game means. You saw on Tuesday the boys that weren't in the 23 trying to push for that Test spot. They played really well and gave the coaches a hard time. Brian O'Driscoll said it after the Brisbane game, 'this is a massive chance now for us to make history'."
Vunipola's Lions Test debut ended in a thrilling win over the Qantas Wallabies but it almost finished on a sour note as the tourists were penalised at scrum time in the dying seconds.
Kurtley Beale missed the resulting kick to ensure the Lions survived but Vunipola knows that a similar slip in concentration could be more costly in Melbourne.
"It was a bit heart in mouth, really. Before that penalty even, when Kurtly Beale missed his first one, I was a bit nervous watching that one as well. To have two kicks missed, hopefully it's a good sign that there's a bit of luck on our side. We're looking forward to this weekend and to making sure they don't get a chance like that again," added Vunipola.
"The three boys that came on, me, Hibbs (Richard Hibbard) and Coley (Dan Cole) all felt disappointed with how the scrums went. The penalty they got at the end, we all knew it was our fault.
"The Aussies got a bit of confidence in their scrum. The players they brought on had a massive impact. (Sekope) Kepu came on and he's a strong scrummager. They added a bit of impetus for them and we didn't stick to our drills and the things that we do well.
"As a young prop, the main thing is experience. Experience shows in the front row especially. They've got a few veterans up there in the front row. Hopefully we'll learn from it and this week come out different.
"We were looking to get out there on Tuesday and thankfully Coley and Hibbs put it right against a strong Rebels team. I was gutted not to be out there with the boys but getting called up this weekend I'll try and set it right.
"There's not been too many scrum sessions - he (Graham Rowntree) was pretty happy with how we went in the first half. Obviously he's done a bit of work with the boys coming on and with myself. He's gone through my clips and pointed out a few pointers we can work on. As a whole scrum, we want to target their scrum again and hopefully get more out of it than last week."