I'm looking for a manga that's similar to Vagabond, Shigurui and Vinland Saga(just to name a few) in terms of both historical and visual realism. The art can be a bit cartoony but not so much that it removes the seriousness of the story and setting. I would prefer the atmosphere/tone of the manga to be dark/gritty.

The first manga that comes to mind after thinking of a historical seinen is Historie. It is pretty awesome and is about a general who served alexander, a biography of sorts
Another one is Cesare it is also pretty awesome

Yeah I love Historie, so awesome.

Mabye Kingdom, I'm not sure if it is all according to history but it is set in ancient china

Perhaps...

Lone Wolf and Cub
The Legend of Kamui
Y十M

Definitely seconding Historie and Cesare, though the tone isn't quite as dark/gritty as the ones you mentioned. I would also recommend checking out Heureka if you like Historie. It's definitely not as in-depth (it's only one-volume!) but I do think it's worth reading for fans of history and Iwaaki Hitoshi.

Billy Bat also fits, but it's much more recent history than the ones you mentioned. Also, while it's historically accurate, its premise is also based on an enormous conspiracy theory.

Wolfsmund definitely works. As does Azumi.

Shut Hell is worth checking out, but it's part fantasy, so I have no idea how historically accurate it is. It's definitely dark and gritty and intensely interesting, though.

Innocent
Alexandros - Sekai Teikoku e no Yume
Sayonara ga Chikai no de
Bokko
Kaina - Surugajou Gozen-Jiai
Yuki no Touge Tsurugi no Mai
Kaze no Sho
Youhei Pierre

As others already wrote, definitely try Historie. Iwaaki Hitoshi's storytelling style is superb.

Agrippa art not so realistic as other ones, but still ok imho.
Otoyomegatari definetely not dark, but the best art you can imagine.

If you can read japanese: Ad Astra

What I like in Trivial Pursuit style. Pick your category:[img]http://i379.photobucket.com/albums/oo233/Reid4891/Manga/trivia-1.jpg[/img]

Thanks for the help, some of these are exactly what I'm looking for. The other sure look cool and I'll be checking some of the out too 😀