I think a simple "Would you like to go out on a date with me" should suffice. That's not particularly vague - and easily done through email or on the phone if you are nervous to do it in person.
edit: and this can easily lead to a conversation about the topic if he has questions about what you mean and needs to clarify what exactly you are thinking you want out of this, or is nervous about going out with you, or doesn't want to go out with you. I also think if he says no, its a lot easier to go on being friends then if you make a big deal out of it and let him know just how much you'd like to "be in a relationship" with him.
Happiness will never come to those who fail to appreciate what they already have.
Last edited by Anneintherain; 01-22-2013 at 06:10 PM.