Vocabulary - well yeah, I kinda learned English majorly in thanks to games. Movies and other things helped but movies had subtitles and didn't require me running to get my dictionary or bothering my parents every time I wondered what the hell the quest dude wants me to do (RPG games like KOTOR 1/2 had a major role in this). I also still pick up a new word from time to time. Like dicktits!

(but seriously, I do

)
Maturity - depends. On the whole I can't say, it's a large medium. Are there examples within the medium that work towards it? Hell yeah. Loads of them. Moral choice games are awesome at this because they often tell you what's "ok" and what's "not ok" to do (ME2 does it in an obvious manner but even other moral choice games let you know whether you're being a douchebag or a saint). Now, whether you're smart enough to pick up some of it or not depends largely on you and your upbringing. To me, yeah, there's been several points where gaming was largely responsible for a growth in my maturity and if you count the community around it (Escapist very much included, not kissing ass, stating facts here), it had some really major (positive) effects on how I view some things.
Does that mean I don't take any immaturity from games? Of course not. But I like that side about me and it never contributes to me being a bad person, just a more fun one to a certain crowd via inside jokes and words - like dicktits!
Intelligence - I'd say yeah again. There's constant problem solving and I don't just mean the classic things. Take my English learning through gaming for example. When I was little and didn't know enough words to understand everything, I had to make sense out of entire sentences based on less than complete information. I would also quickly pick up when developers ordered the answers in a certain way (first answer good, second answer neutral, third answer bad, though usually more complicated than that), thus allowing me to pass through certain dialogues the way I wanted to without even fully knowing what I was saying and what was being said to me.
Moreover I've learned to quickly pick up on some developer habits. A scene comes to mind with me and a friend playing in WoW. We were leveling in Stranglethorn Vale and had the quest to kill the stealthing panthers. As we couldn't see them until we were close enough to be attacked, it was supposed to be a (obviously slight) challenge to find the damn things. However, knowing the general area where they hung out in, I actually didn't have any trouble literally pinpointing them down with pretty immaculate precision.
Why? Cause I played enough games (and made enough shitty maps for some games

) to know - developers will fill an area with mobs in these situations. If they're stealthing, just look for where there is nothing at a certain (what I'd call 'optimal') distance from the nearest creature (and possibly some bush/tree/etc. they could visually "leap" from for extra surprise element) and the damn thing's gonna be there. Look for creatures you can see then just go/attack right on the point where it's far enough for the stealthing mob to be in order to be spaced out enough.
There's a lot more examples obviously, but yeah, I'd say in general they do increase mental agility and reflexes for some situations and possibly even hand-eye coordination (though fuck me if I can play darts for shit even after 15+ years of gaming). And of course. There are always. Dicktits

(yeah I get it's silly/immature/whatever, but just felt appropriate as a juxtaposition to the rest of the post and a bit of an ongoing joke

)