Weeping and twisting vines don't drop every time

They do drop, though they drop less than they should.

they have 33% drop chance

Can confirm. Affects weeping vines too. Probably intended.

They are set to have a 33% drop chance in the loot table, so I would say this works as intended:

"type": "minecraft:item",
       "conditions": [
                {
                       "condition": "minecraft:random_chance",
                       "chance": 0.33333334
                }
 ]