Traditionally, a third, civilian Vice Chairman has only been 'elected' towards the end of the current Chairman's time in office. Their election is a strong signal that they will succeed the Chairman, and thus become the most powerful man in China. For generations, it has been obvious by looking at the Politburo Standing Committee (and accompanying political and analytical consensus) who the next General Secretary of the Party will be, and this individual could be assumed to in time become Vice Chairman of the CMC.

However, since Xi has upended the previous system of technocratic succession, no such candidate is identified. Thus, an unexpected civilian Vice Chairman would be a strong signal.

'Civilian' in this question means someone who is a career party official rather than a career military officer. It is entirely possible that, like Xi, such a figure will have done a stint in uniform. That is not the intended meaning here.

New 'military' Vice Chairmen are a possibility, and the political significance would be much less, thus this question is not occupied with them.