I know I'm a little late here. But I thought I'd add in my two cents on vocabulary. In SZJMB one of the characters is a pint-sized genius and he talks like it. I didn't worry about it too much, because you weren't really suppose to understand what he said and the other character softa translate for him when needed.
Turns out he's the most popular character by far.
Also, I have my MC use big words now and then (he's a smart kids who pretends not to be). I figured it wouldn't be an issue, because the reader can usually gather the meaning from the context.
Even though SZJMB was it suppose to be a reluctant reader, I haven't had any complaints about the vocabulary. In fact, I've had more younger readers than I expected (8+). So honestly, as long as the words fit the MC, I doubt I'll ever worry about vocab. again.
Now, it might be diffident if you write in the third person, or if you tend to have an advanced vocabulary. But, in the first person, if you can imagine your MC saying it, then it's probably fine.