From Vinland Saga World Facebook page: "To be honest, I wasn’t familiar with this leaker at first. However, according to several users, this account is actually OECUF / SUGOI LITE himself — a name many of you have probably heard before, as he’s widely considered one of the most reliable anime leakers, with a very high accuracy rate and very few misses. Yesterday, on what appears to be another personal account of his, he opened a Q&A session, inviting people to ask questions about what he knows. This is what he mentioned regarding Vinland Saga. Interestingly, this aligns with the frequent Vinland Saga illustrations that Takahiko Abiru (the anime’s character designer) has been posting on Instagram since September, which could suggest that production may have picked up more momentum around that time. For those unfamiliar with why the anime has taken so long to continue, the main issue was that the core staff was busy working on other projects, such as Oblivion Battery, Inazuma Eleven, and Ranma. The production committee has always wanted to continue the anime, but only if the same main team from previous seasons could return.  
It seems that issue may have been resolved — or, as some suggest, there could be changes in certain positions, such as the series director. (Atsushi Kobayashi is a brilliant director and was responsible for some of the best episodes in the entire series.)"

Season 3 is in production, according to a reliable leaker