Which doesn't belong? Ecuador, Kenya, Borneo, Congo, Vietnam?

My sons home work asked which of these doesn't fit with the other 4. for the life of me I can't figure it out. The sample question from the page was Brazil, Peru, Ecuador, Chile, Panama. Panama did not fit because all the others were countries in South America. I just can't seem to find the answer to Ecuador, Kenya, Borneo, Congo, Vietnam? Thanks for your help in advance.