Being vulnerable to someone is not telling them all the things your heart has been hiding. It is not scaring someone with the truth about
how you feel. It is not necessarily:
I have thought about you every day...
You are the only person I think about...
I love you...
I want you with me, always...
I can't live without you...
When you believe those things to be true, and they burden you and plague you no matter who knows them, how is that laying yourself open to be vulnerable to someone else? That is merely laying yourself open. You have made yourself what you think is vulnerable, when it is really just a one sided expression that was never meant to be one sided. If it is, it isn't complete, it isn't love. It is an infatuation, a crush, a meddling set of emotions that only weigh you down and make the other person feel guilty and helpless that they cannot be returned to you.
No.
To truly be vulnerable is to hold those fancied, flowered thoughts in your mind because they are valuable enough that you thought them up and don't require an outside validation to be true, to be real. When they become things you smile about when the other person is near and they ask you why you're smiling and you don't answer them, it seems like you're being pompous or taunting, but let that pass.
Being vulnerable means not always knowing, and not having it be the end of the world that you don't always know. It means to hold your tongue when you'd rather spill your guts. It is the discipline to withhold so that the other person isn't scared away by your intensity. Those things you think, they are valid and true, but they should be things you say sparingly after things have been established. When you have that person in your arms down the road, and you've spent a good portion together, that's when you should say those things.
Being vulnerable means taking words only to the level that the other person is comfortable, and not breaking over thinking that you are not causing more chaos than the release of words is worth. It is putting the other person first. That's the only time when you are letting someone have a chance at hurting you.
An addition
Another way that I would consider being vulnerable to someone is derived from plans that go awry. I realize how much someone has an effect over me when we make plans to do something and for whatever reason they fall through. Even if the reason the plans fell through are not the other person's fault, I take it personally. I realize then just how much it mattered to me that I couldn't get to spend time with them or see them.
It sinks my stomach. I sit there wondering why I'm still sitting there, waiting. You have no clue at this moment why I am waiting, because, well, because you're not here. You were supposed to be, and now my proud, supposed self-dependant sorry ass is going against every ethic I have and sitting here staring at the phone. Does this make me psycho? Does it scare you that I can't just blow off that you blew me off? If you say I think too much, I say you don't think nearly enough to have other people co-exist in your life. You suck.