The Mongol Invasions of East Asia??

I recently talked with THE PrinceOfMacedon, and we were talking about how terrain effected invasions. I asked him how the Mongols took down China back in the Medieval times and Korea. But my original question was that how was Vietnam able to defend against a near full scale invasion of the Mongols, and then Prince told me that Vietnam's terrain screwed up the Mongol's "fluid, horde tactics". Is that really true , that my countrys guerilla warfare and terrain defended against 3 invasions from the most Powerful Military force in the world