Vanilla vines not spawning on custom trees

I have been searching for this for a while and can't seem to find an answer. I have created a biome with custom trees and selected to have vines spawn but when I select the vanilla vines as the vines they don't spawn. When I select a custom block to be vines then viola, the custom block covers the tree. So why are the custom blocks working as vines and the vanilla vines are not???