Vice and WPP's media investment unit GroupM have signed a multimillion-dollar advertising deal that will take advantage of the millennial-focused media company's international expansion.

"We both agree that millennials are the future," Vice co-founder and CEO Shane Smith said in an interview with CNBC alongside WPP CEO Martin Sorrell. "They're under-serviced and we have to create more content and do a better job of going after them."