It depends on the context. If I say, "Jon has been a vocal advocate for the needs of military families" that would be a commendatory. But, if I say, "John is unreasonably vocal in his opposition to veteran's benefits," would be mor derogatory.

This wholly depends on context and situation. For instance: "I appreciate you being very vocal. This way, I never have to guess what you're thinking." This is commendatory. The speaker likes vocal people because the speaker doesn't want to try and figure out what is actually on someone else's mind. "You were so vocal in the meeting that we spent too much time listening to you instead of the boss." This is obviously not a positive comment. You ended up taking over a meeting that the boss had a different plan for. I hope this helps answer your question. Please let me know if it doesn't (or if you prefer receiving your answer in Chinese).

To be vocal means that you are willing to speak out your beliefs on a certain topic. It's usually neutral, but it can easily become an insult. For example, someone might say that you're vocal, but they really mean you're TOO vocal.