It is very rarely you find a piece of art like this where every aspect is done at such a high level that there is not much needed to be said. In terms of manga, I have only seen a few other series that do it to the same level as Vagabond and even then, I believe Vagabond stands at the top. Takehiko Inoue is a master at work, his art is second to none, his character writing is matched by few, and his storytelling is wonderful. The only flaw you could give is that the series does not have a definitive end and there has not been a single chapter released in the past five years, but even then, it is not too large a flaw. Anyone who has read Vagabond knows that where we left off is quite satisfactory, of course I would love for Inoue to come back and finish the series but I do not think it is absolutely necessary. Where the characters have been left off, especially our main characters Musashi and Kojiro for which we have seen plenty of content to be happy. To go back to the art, you see why this series as a manga is so vital because of the visual storytelling that Inoue does. I have not read the original novel that this series is based off of nor do I know how similar these people are to their real-life counterparts but I don't think either of those could give me the same experience as Vagabond does.
Author Inoue, Takehiko