The bug

When used on the weeping/twisting vines, a fortune tool can cause a single vine to drop up to 7 items, which can then be placed and broken again, essentially duplication for free. Expected would be that fortune increases the likelihood of vines dropping.

With fortune 3, you get a 33% chance of getting 1-7 plants, which averages to 1.32 plants per plant.

How to reproduce

1. Put a few twisting vines in your off hand and pickaxe with fortune 3 in your main hand
2. Start spamming right/left click
   →  Free vines