Visa is required for citizens of Mongolia to visit Vietnam. Mongolian nationals may have 2 options: Visit Vietnam embassy in Mongolia or apply for a visa on arrival to Vietnam from Mongolia online. Mongolian citizens may apply for both tourist and business visa to Vietnam for the stay of 1 to 6 months from Mongolia. Please check this link for visa fee for Mongolian citizens.

All the information for getting a visa from Vietnam embassy in Mongolia can be found here: http://www.vietnamembassy-mongolia.org/