Vegas Pro is incredibly user friendly compared to it's more technical counterparts of Final Cut or Adobe Premier Pro. I've started using Vegas since 2011, took a break and came back to again only to realize how much I loved it in the first place.
I can't say for sure how well the tutorial videos on Youtube are like. It has taken me awhile to learn more of the complicated stuff I needed to learn to make something. But it's still possible to find the high quality tutorial videos for Sony Vegas. It's just not as easy as Adobe though. But is it better then IMovie? I'd like to think so.
How long would it take you learn what you need to learn for simple effects and basic needs for gameplay videos? 1-3 months depending how consistent you are I'm guessing. For more of the complicated stuff...3-12 months. Regardless though however long you spend in Sony Vegas Pro I know the editing power and results will be worth it.
Best of luck to whatever you decide though. An editor is just a creative tool for you show to yourself and what art you can make. So whichever one works best for you is the best answer. =]