by 3d vine blocks i mean, Cave vines/Glow berries, Twisting vines and Weeping vines, by standing on top of them I mean relatively on top (image for example)
(https://user-images.githubusercontent.com/68875386/146688871-4142ee7e-5483-4342-b732-fa5f5be21089.png)
If you try to use them from beneath they don't work

That is how climbing works in minecraft, only the block your feet position is inside is checked, you can see the same behaviour in vanilla if you try to climb a ladder that is on your eye level, you won't be able to unless you jump.