You need to understand where your co-worker is coming from and why he's being vague.
Perhaps he's the type who believes knowledge is power and therefore he hoards his knowledge. In this case you need to figure out how to reassure him that he's not making himself vulnerable by giving away details. And before you ask how to do that, it's up to you depending on the type of person you're dealing with.
Perhaps he's incompetent and is only capable of giving vague answers. Get your manager involved, if he/she isn't already, and in a professsional fashion identify that there's a problem and request intervention.
Perhaps his communication skills are lacking. This may be his only way of communicating. You need to find a way to get him to accept some kind of training.
Perhaps he's egotistical and his intellectual superiority compels him to be vague because he smugly believes it's your job to understand him, not his job to make you understand. In this case you need to meet him on common ground, showing him you're just as (if not more than) intellectually gifted.
The overall point I'm making is that there are a wide variety of reasons why this person's acting the way he is. Whatever the reason your best approach is to get into his brain and understand why he is the way he is. Don't fight him, work with him. That will help you determine the appropriate course of action.
