I didn't acquire them completely legallySyelraedhel, how are you watching the past Vikings episodes? I'm only aware of Hulu and Amazon Prime, but they are for a fee.
Thistle, Vikings is awesome! Start from the beginning and don't skip any episodes!

And yes Thistle, I totally recommend the show. For a Michael Hurst project, it's not as historically inaccurate as you might expect! (There are still quite a few jarring inaccuracies, but none really that TRULY break your immersion in the story. And if someone was not read on the period, they probably wouldn't even know).