You can start by getting familiar with the characters and the story background. Then, pay attention to the details in the panels and the dialogues. It helps to read it in a quiet place to focus better.
Yes, Vinland Saga is based on some historical elements. It draws inspiration from the Viking Age and the Norse exploration. However, it also contains fictional elements for the sake of storytelling, like the personal adventures and relationships of the characters. But the backdrop of Vikings sailing, their culture, and the idea of a land like Vinland are rooted in historical research.
Sure, the Vinland Saga manga is complete. It took some time to wrap up, but now readers have the full story at their disposal. Different people have different opinions on how well the ending tied everything together, but it's definitely worth checking out.
The Vinland Saga manga is excellent. It combines historical elements with intense action and emotional depth. The world-building and character arcs are very engaging.