Saracens and Leinster clash in Newcastle, with the duel between the front rows a key battleground as the English champions attempt to dethrone the current rulers of European club rugby.
The pair had little first-hand knowledge of each other until they were selected for the 2017 Lions tour to New Zealand and went on to start all three Tests of the drawn series.
Vunipola has since established himself as the game’s outstanding loosehead and the England star insists it was Furlong who showed him what a prop can achieve.
“Tadhg has proved to be one of the best in the world, if not the best,” said Vunipola, who is set to be restored to Saracens’ front row after recovering from an ankle injury.
“Hopefully we can nullify some of his impact, but you can only do so much. I am looking forward to a great match-up.
“He surprised me on the Lions tour in the way that he was such a big guy and his set-piece stuff was solid, but around the park I couldn’t believe how hard he worked.
“I thought, ‘I’m all right at this’, but then you saw what he did and you are like, ‘Oh, no I’m not’.
“It gives you motivation to push yourself and that is what you get with good players.
“That is one thing I take from those tours and being around those great players – you are always going to learn, it’s just a case of whether you want to.
“He has got all the skills and sometimes you forget he’s a prop forward, but that never takes away from his bread and butter, which is his set-piece. Everything else is a bonus.
“He’s very reliable as a player and when he trains he always gives 100 per cent. That is the sign of a great player in my eyes.
“Off the field we were close. He’s a top bloke and he’s very easy to get along with. Front-rowers tend to migrate towards each other anyway. We spent a lot of time together on the Lions tour.”
Saracens were succeeded as European champions by Leinster last season and were well beaten by the Irish province when they met in the quarter-finals.
While the defeat was a bitter pill to swallow, Vunipola has revealed that it ignited their march to the Gallagher Premiership title.
“It did hurt a lot. It gave us a kick in the arse for the rest of the season and opened our eyes a bit, in that we probably thought we were something that we weren’t,” the 28-year-old said.
“We had to be honest with ourselves, so it was probably the best thing that has happened to us. Leinster showed us there is another step up. It was great for us.
“But this weekend is a new slate, so hopefully we can go out there and put in a better performance than we did that day.”
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