That depends on his position. Chairman, vice-chairman refer to a political body or a company's board of directors. Vice president as Randy said is the correct term; at least in the US. There can be many vice presidents in the same company. Director is another generic term you can use. Unless his position is clearly indicated (which you have not), you may be slighting your boss by using the word, "vice". So I "director" is much safer.

A company would not have a vice-chairman as an operating officer. Saying vice-president would be better.

Without giving to many public details, you can say, "acting XXX" if the vice president is acting for the duties of a XXX.

Dear Jeff, thank you for your reply. The person I'm referring to is the vice president of the management board of the company. Actually he's not an operating officer but he is nominated by the owners for a limited period of time.