Mongolia to Beijing to Vietnam

I am a Canadian Citizen, I will travel from Ulaanbaatar Mongolia to Beijing, I would like to stop for 72 hours then fly to Vietnam. in the language for the 72 hour passport one requirement is (Visa for a third country or region) When travelling to Vietnam you can do it on arrival. Will this be an Issue? I would be able to provide my application form for the VOAfor Vietnam but will not yet have the stamp. Can I get the 72 hour free transit?