What is the difference between bún tươi and bún khô?
I'm going to go out on a limb and say that there doesn't seem to be a difference in the two. They are both rice vermicelli noodles. 

While I found very few search results in English, they appear to be the same. I also searched images and found that there are packages of each but there are also packages labeled like this:



So I came away thinking it's more semantics than anything.