72 Hour China Visa - arriving from country not on list of exemptions

Hi, We are flying from Vietnam to Chengdu and then on to London 2 days later. We hold UK passports. I think we qualify for the 72 hour free transit as we are arriving from one country and departing to another but wanted to check as the country we are arriving from (Vietnam) is not one of the listed exemption countries. Any help greatly appreciated !