Vinland saga is only 8 hardcover volumes at the moment, it's pretty easy to collect honestly.

But with completed series I'd recommend:

Lucifer and the Biscuit Hammer - One of my all time favorite series. Unique characters, an original tone, and it manages to have a great climax (which I consider kind of rare in the medium). You can get the whole series in five 2-in-1 volumes.

Pluto - A retelling of an Astro Boy arc, but done in a more realistic and noir style. This is Naoki Urasawa, perhaps the greatest writer in the medium. You might know him from Monster, which has a solid anime adaption. He has another excellent series called 20th Century Boys, but it is pretty long so I'd start with Pluto. Pluto is complete with eight volutes.

I Am a Hero - This is a bit of a cheat, but technically the series ends in a few weeks. It's a Zombie story, but has a very unique main character and a feels different then a lot of series in the same genre. It also starts slow, I'll readily admit that. There are three volumes out in English at the moment.

I can stop there, but I'll also mention there are a lot of great manga that have poor or incomplete anime adaptions. Soul Eater, Rurouni Kenshin, and Akame Ga Kill all come to mind.