It is known for many years that high doses of vitamin B12 can cause acne on the face and upper body. This usually happens when one gets a megadose vitamin B12 supplements and with symptoms promptly improved when the use of B12 was discontinued. That being said, vitamin B12 is important to your health. Thus you should definitely continue to take it if you are vegan but avoid megadoses. Consult with your doctor or dietician about the exact quantities (and forms) of B12 you should to take for optimal results.