When a vine is placed directly on the underside of a block, its texture will be oriented to the north. However, placing additional vines in the same block space as it will cause the ceiling vine to rotate, which seems a bit odd.
This is due to the vine block using several predefined models, which are rotated accordingly. This could be fixed by instead using a multipart model for vines, which would bring it more in line with other blocks with directional states, would reduce the amount of model jsons needed to define the block cutting down on space without sacrificing aesthetics, and would also fix an issue where vines with all states set to "false" appear as though attached to a south face, rather than being completely invisible like with completely false wall blocks.
I can confirm for 20w46a