(Mild spoilers ahead)
Well, It's indeed a saga.
I will start by saying that anyone that wants to read this manga must be prepared for some level of dissapointment, not because it's bad, because is a story that has many changes and that can and will alienate a lot of people.
The overall story, at least without the last arc that is still in progress, can be split in three parts that are almost completely tonally and thematically different. The first one is a revenge story about a young viking that wants to kill on a duel the man who killed his father. This part is actiony and has some really profound characters and tension thorough, so evidently is the part that most people like and that becomes evident when the next part arrives.
The second part is almost a 180 degree turn to everything that have been done until then. Instead of a viking, the protagonist ends up as a slave and the fighting is almost reduced to zero. But the thing is that the focus changed from a historically based story about vikings to a growth story about the protagonist searching a purpose, thinking about what he had been doing during the past ten years and following his father footsteps to be a warrior that does not need violence.
A lot of people find it boring and some find it ridiculous considering the time it's set in, that there's no way such a pacifist ideal can work in such a rough time but the story itself recognizes that fact and that's way the protagonist goes through some really hard times to stick to hs ideals and still be of help to the people around him. There's moral dilemmas, some deep introspection and lots of character growth. It's a great part, but evidently, the pace of the story is a lot slower and the lack of action added to the always prsent but now finally applied pacifist ideals alienate the people that started reading for the battles and the war themes.
The last part, is even more alienating that the last one but for different reasons. After the things in the Farmland arc, Thorfinn ends up crossing the baltic sea with a certainly curious group of characters and ends up involved in a war for the leadership of the strongest group of vikings of the time, the jomsvikings. The thing is, that this is clearly a seinen series, evident mostly for the second part, but even that changes when this arrives as it gets more and more shonen characteristics that progressively changes the tone again until is completely different than the other two parts.
I will say, it's certainly a decay. Various new characters are introduced but they are not nearly as profound or developed or interesting as the previously presented ones. Comedy, until that point basically null, becomes fairly common and even the art starts shifting to favor it which makes the serious tone that it had until that point dissapear completely. There's even certain character that was pretty interesting although half-cliche in the first part that comes back and becomes completely cliche and basically a comedy source. In fact, with him there's at least three characters that fill that role and even could be said that a group of characters is almost like a team rocket level of villains. Certainly, is a lot cheaper, easier, less serious, less nuanced and a way less refined story that was being presented until then.
But, in it's favor, I will say that this part is a fucking blast. It becomes basically a shonen, but I will admit that a pretty fun one at it. The comedy, although breaks with the tone of the past stories, is actually funny. The events are interesting, the fighting and war come back and now that the protagonist is following an pacifist ideal figuring a way to get him and all his friends out of the war is certainly a interesting challenge to follow. There are various groups of separated characters doing different things and at the end they all end up contributing to the events so the story and the war progresses. It's certainly an inferior story that the other ones, but again, it's a fun story at least.
Overall Vinland Saga is a really good manga. Certainly decays in the third part and that must be taken into account considering the type of story this is. Is not perfect and certainly someone that is in not for the overall story or to follow the protagonist will be dissapointed at some point in some degree, but even with everything I would say that it deserves a try. Is evident the author has some real talent and it has a lot of passion.
To end it, I will mention that the art is really good, it improves a lot in certain point but becomes simpler in the third part to favour the new tone. So even that is changing, kind of fitting for a story all about the changes and growth of it's protagonist.