I can only speak of my experience. When I played video games, especially first-person-shooters, I developed better reflexes. I am not sure that I still possess these reflexes but these types of games can definitely help to improve them. The adventure games can help in a different way.
Since they have a story, you need to understand it. Subtitles can help you to learn about the objectives and tasks so you can improve your English. These types of video games helped me a lot to extend my vocabulary and even my grammar.
But I do not think that they can substitute a dictionary or a grammar book or a teacher, of course.