How would the sentence stay the same if vulnerable were changed to open ?

The meaning will be subjected, but still equivalent.

Explanation:

Since there is no specific sentence, allow me to offer my own example:

Sylvia refused to become vulnerable.

See? Here, it is apparent that Sylvia does not want to show people her soft side. But, see how it differs in this sentence:

Sylvia refused to become open.

Now, there is a tonal change. When you are vulnerable, you are willing to admit that you are afraid, scared.

But open, in a more of a subversive manner, means the same thing. But open means you open up, or reveal yourself to people.

These sentences are similar, but they can be interpreted differently.

Hopefully this was helpful to you.

just say it's your friend did it and you had no idea?