In mine Vila is because he was born one and says so, but it's obvious he's very intelligent. I have him turning down a promotion at 12 because he'd have to work hard and leave his mother and he's not willing to do either.

I also don't think he's lying about the Space Fleet training he didn't fancy doing. He's actually very good at what he does on both ships and could have made captain, but then again he's allergic to responsibility. :-)