The All Blacks are currently eyeing their third successive title in the showpiece event after previously winning during 2007 and 2011 editions. However, it was a mixed year for Steve Hansen's men that saw them defeated by South Africa and Ireland while England came close to beat them before succumbing to a one-point defeat.
Vunipola, who missed out England's autumn international series due to a calf injury said Steve Hansen's men are still favourites for the 2019 World Cup in Japan but have lost their invincible tag.
"The gap is closing with New Zealand, but it's not just about them, you can see how every game internationally between the big teams is a proper contest," he told The Rugby Paper.
"That's what you want. New Zealand are still World Cup favourites because they still have that experience of being number one and they're still the team to chase, but England competed well and you can see how Ireland have consistently challenged them.
"Ireland have beaten New Zealand twice in the last couple of years and shown that they are human.
"Although it's still a very big task, England proved they can go toe-to-toe with them as well and we can improve again between now and the World Cup".
England had a tough 2018 year where they finished fifth in the Six Nations title followed by a 2-1 series defeat in South Africa but managed to revive themselves during the November series where they won three out of the four games.
"The boys did really well in November. There was obviously a bit of talk about how previous results had gone but for me it looked like a new team out there.
"It looked like they played with freedom and you could see an identity coming as well. That was the pleasing thing and also, for me being on the outside of it, it was exciting as well because you want to be part of it and know that's going to be a challenge."