So, I'm completely new to League of Legends - haven't even touched it yet. What kind of gameplay can I expect out of it? Is it the inspiration of Vainglory? Is it some type of Battle Arena game?
It's a MOBA that's more fast-paced than the likes of DOTA 2. It's fun and challenging in it's on ways, but the community is pretty toxic and you'll probably have more games with nasty players than without. Although it's easy to load in the game and just /mute all. If you like improving yourself and carrying teams, check it out as long as you don't mind the toxicity.
Absolutely. It's not too popular in NA but the other regions are still growing. I think EU just passed 3 million users, and the recent semi-final match set a record for viewers.