The regular overworld vines which grow predominantly in the jungle and mangrove swamp biomes will only drop as items when mined with shears, and will not drop themselves when mined with any other tool, even if it has silk touch. This is inconsistent with the Nether vine variants, which both have a chance of dropping regardless of tool, and will always drop with silk touch. The chance to drop without a tool seems to be an intentional feature, but it seems odd that the regular vines don't drop with silk touch.