Who is a "Vowel" in a panel of Jury for a Master's Defense?
"Vowel" is likely a translation error from Portuguese, Spanish or another Romance language - probably caused by Google Translate.

In a committee or a board, a "vogal" or "vocal" is a member without a specific role. For example, in a committee I was recently to grade end-of-degree dissertations, the three members were the president (tasked with chairing the committee), the secretary (tasked with writing grades down) and one "vocal", with no specific task besides deciding grades with the other two members. 

Another meaning of "vogal" or "vocal" is vowel. Both meanings come from the Latin word for voice or speak, because a "vocal" is a person who just speaks in a committee and vowels are sounds or letters that can be pronounced alone.

I tried to find a translation into English but no dictionary gave me a suitable one. Then, I would translate "vocal" to "board/committee/jury member". Maybe boards are organized in a different way in English speaking countries.

Just for reference, definition of vogal in Portuguese (see meanings 3 and 4), and vocal in Spanish (meaning 5) and Catalan (meaning 2).