Your remark about wanting something special with this woman and knowing that you risk some pain is very alarming.
What kind of pain are you anticipating?
Before you go farther, you need to identify what is nagging at the fringes of your consciousness. On some level you know what is going to happen to you here. It almost feels like you are about to lauch into a repetition compulsion ( see my Advice Archives) in which you will play out an early trauma and suffer terribly.
The fact that you are anticipating being hurt is my clue that there is something that your unconscious mind knows about this woman or this relationship. The only way your mind could know this is because you have been down a similar road before. There is a script in your head, and you know the ending before you start.
So, find out what you may be playing out here. And, ask yourself how you foresee being hurt. What do I see playing out?
You sense that this woman is attached to another man and that she is going to either leave you or two-time you, or perhaps resist becoming involved with you. If this is what your fear is, then watch yourself like a hawk.
If you've been reading my columns for a while you know that we humans unconsciously recreate the traumas of childhood (repetition compulsion) in order to work for a happy ending. There is something that you know about this girl that you don't want to face.
You also seem to know that pinning her down as to how she feels about him, you, and becoming serious with you will be dangerous. You know on some level that this girl is very ambivalent about becoming committed.
Before your heart is in a noose, identify your repetition compulsion. Find out what happy ending you are looking for. And, take an honest look at whether or not this girl is capable of giving it to you--your happy ending I mean.
Examine your own fears of intimacy (one way to stay safe from commitment is to choose a partner who holds back or can't choose between you and another lover, etc.).
Ultimately, you will want to open a discussion with her and find out exactly where she stands with you and this other guy. To leave this in the vague, and remain silent, may be the way that your unconscious mind intends to rope you back into another painful repetition.
If you don't talk to her, you could find yourself deeply involved with a woman who loves another and is unable to commit to you. So, identify your issues and take the bull by the horns and talk.