And he has gone from strength to strength this season, with Saracens currently top of the Premiership table.
"I feel a different player a year on," Vunipola told the Evening Standard.
"It's been a great experience coming into a new environment with England and then going on tour with the Lions.
"I feel everything is a bit more familiar in terms of coaching and training but if I rest on my laurels I will be left behind because we have players coming into the squad all the time and I have to keep my head down, keep training and try to impress the coaches."
Another hero of the Lions tour was loosehead prop Alex Corbisiero, and Vunipola believes England now have strength in depth in the position that can only help the team going forward.
"There is a great group of loosehead props with Alex Corbisiero, Joe Marler and Matt Mullan and we are lucky to have so many players and we all help each other out," he added.
"I think I get on well with Joe because we are different personalities and I have known him since the England Under-18s and we do like to enjoy ourselves.
"I have known him from the time when he had long, sensible hair and no beard all the way through the different styles and various beards.
"He has had a few dodgy hair cuts over the years but that's Joe for you. He is a great laugh and a really good guy."