No. Vinland is not a part of the main story. While it does help the player unlock a fabled 100% completion rate and additional achievements/trophies for truly stopping the Order, it is entirely optional and hidden. It is more a reward for thorough players and those seeking a challenge than a slyly hidden "Oh wait! there's more!" kind of deal. Ubisoft wanted to reward players who already experienced Vinland in the series's third installment and to reward players who left no stone untouched. Vinland is the type of secret that gamers talk about between each other. It's the kind of playground story gamers my age experienced when hearing about the hidden 4th/Tofu Survivor modes in the original Resident Evil 2. It's the type of thing that makes me contemplate starting a brand new save and giving it a shot. Perhaps it will motivate you to do the same.