Simmonds, who made his Test debut against Argentina in November 2017, is 12 kilos lighter than Vunipola. But England forwards coach Steve Borthwick predicted speed would compensate for the lack of bulk.
"Sam is quick off the base of the scrum, he finds space when he carries and is very hard-working. He has some real ability and has a great attitude to go with it."
But for all the pre-match plaudits, Simmonds will have to prove his brilliance against his opposite number – the Italy skipper Sergio Parisse who is widely acknowledged as one of the outstanding No8s in world rugby.
"Of course there are differences between Simmonds and Vunipola," Parisse said on the eve of the clash at the Stadio Olimpico in Rome. "In terms of carrying, maybe in terms of physicality, England have lost a little bit. But I'm really sure Simmonds will want to have a big game.”
England are bidding to become the first team in the 18 year history of the competition to win three successive titles. By contrast, Italy have not won a Six Nations match since a 22-19 success away to Scotland in 2015 and have lost all 23 times they have played.
"I think everyone in this Italian team believes in the work we are doing, so of course the results are very important," Parisse said. "We don't have any pressure. We are playing England who are the second best team in the world. They won the Six Nations last year so they have all the pressure.
"We are convinced we can put in a big performance against a great team. As for the result, we'll see at the end. But the important thing is to prove ourselves against a great side."
Borthwick, who played in England teams that scraped 23-19 and 17-12 wins in Rome in 2008 and 2010 respectively, said past successes were irrelevant.
"This is international rugby and these games are always intense encounters and we're expecting that intensity against Italy," he added.