Help With Custom Vines

I need help creating custom vines for a custom dimension. I have custom trees, (NOT made from structure blocks, I wrote them with procedures), and I want my custom vines to randomly generate on the trees and hang like normal minecraft vines. I have made it so when a vine is placed by a player, it places with the proper rotation, flat against the tree. I just can't get them to naturally spawn randomly.

PS: Growth isn't really important to me, I just want them to randomly generate on the trees. 

Any help is GREATLY appreciated!