In February 1D's Niall Horan revealed on Twitter he was working with Justin on new songs, while Harry Styles was a special guest of The Vaccines when the London rockers played at The Brit Awards War Child gig this year.
In an exclusive interview with MTV News before their huge O2 Arena headline show, Young said he understands why some Vaccines fans are disappointed about their collaboration, but the likeable frontman insists he "really enjoyed" the experience.
"They are both natural writers," said Justin when we asked him about his recent studio time with teen dreams Niall and Harry.
"I love writing songs and love pop music, so the process was exciting. I don't think they [One Direction] are going for a heavier, indie sound.
"Maybe it's damaged my credibility, but it wasn't The Vaccines who wrote for them, it was me, so I doubt it's damaged the band's."
Since making MTV's Brand New For 2011 shortlist, The Vaccines have become firm festival favourites and in September topped the UK album chart with their second record Come of Age.
Backstage ahead of their O2 Arena headline show, birthday boy Justin admitted he is still coming to terms with his band's success.
"It's insane because our first-ever gig in London was under three years ago, so to play here is mind-blowing.
"It's massively surprising how far we've come so soon, because honestly my aim was to headline the Barfly with songs like If You Wanna and Wetsuit, so it's crazy."
And Justin revealed to us The Vaccines are hoping to release their third album next summer.
"We've just recorded a new EP in LA so we're writing and I'm feeling really creative at the moment, so we're going to come off the road in October and by this time next year it'll be recorded."