Bord Na Móna took over as distributor for the brand in the UK in early 2012.
Harper said: "Given the success Vital Earth has enjoyed both as a brand and as a company, I know that I am leaving it in good shape for the future, and that therefore the time is right to do so. I am looking forward to a new challenge and to making an equally positive impact with another business."
Harper has yet to take up a new position. A successor has yet to be appointed.
News of Harper's departure follows the appointment of Peter Rush at growing media firm William Sinclair, Rush has replaced Bernard Burns as managing direcotor.