Vagabond is a documentary in a way, in the same way as Kingdom, so we already know its ending, so the absence of an conclusion impacts less
Regarding the comparisons, I would like to separate:
With Kingdom and Vinland, we have the role of adapted real history, it may not be the literal story, but we see in the works an attempt to portray the story of a character and/or people
With Blade, the "reality" of sword fighting and the life of a samurai
With Berserk, I believe that the biggest comparison is with absurd art. But we can also remember the legendary arch of the golden age, which in my opinion is the best manga about knights or medieval era ever made.