I should haveread this question before answering the other one. If your vice chair simply made the announcment got up and walked out, then it would amount to serious inapropriate behaviour and could be considered to have brought the council in to disrepute. That would likely to be a breach of the Code of Conduct.
As i mentioned in my other reply, the chairman (or vice-chairman) must take the chair if present. There are only two means by which he need not. The Chair can ask to be excused the responsibility (perhaps on the grounds that he needs to leave the meeting early for example) and the council can vote on the question. The other way is for the Chairman to resign as Chair with immediate effect.