Basically, visitors to Vietnam must obtain a visa from one of the Vietnamese diplomatic missions unless they come from one of the visa exempt countries.
On default, you will receive an invitation letter from the IWAMSN 2018 organizer via email and in general, such an invitation letter is sufficient for visa application. If you needed an original printed invitation letter with an offical stamp, please complete the visa form (*.doc or *.pdf) and send it to the conference secretary.