Having only made his England test debut against Fiji in November, he then played a part in all three of the Tests against the Wallabies.
And despite himself having to battle for the same position as the Lions winner, Gill believes the experience gained will benefit both club and player - even if that does mean putting up Vunipola's new attitude.
"He's a lot more arrogant," joked Gill. "He's gained a lot of experience. Playing at that level, you would.
"He went with the British Lions in the summer, he's got great potential and he's a great prospect.
"Being around those sort of guys is a great experience for him at that young age. He's come back and he's trying to put what he's learnt there into our game at Saracens.
"It's good to have someone fighting for your position. We've got rotation at the moment, play two games, rest two games, so when you get a chance you've just got to try and play as well as you can and try and keep that shirt for the big games."