I like my grilled cheese how I like my Vietnamese villagers, yellow and melting
Even for a joke, that is horrible to say. What are you implying about the Vietnamese or the residents of Vietnam?
It is just a joke about the skin color of Asians
It is a vast generalization to believe that all people from Vietnam have yellow skin. If you look at people specifically from Vietnam, you will see that they have skin that varies greatly in color just like all people do.
I guess you are right, since I don't know much about Vietnamese people.
If that's the case, then please refrain from saying such harmful stereotypical jokes that can offend Asians. Since you said you're not very knowledgeable, why not take some classes on Asian American history? It's a wonderful start on learning about all of these amazing people.