The certificate of Vietnam visa exemption might take the following forms:
1. A stamped certificate: A visa exemption certificate is stamped on the passports of Vietnamese nationals living abroad and foreign nationals to allow their entrance into Vietnam.
2. Detached Certificate: A visa exemption certificate issued to Vietnamese people living abroad under permanent resident’s permit granted by foreign authorities and holder of passports of foreign countries or territories with which Vietnam does not have diplomatic relations.
Other information about Vietnam visa requirements and Vietnam visa exemption can be found here http://www.vietnam-visa.com.