You cannot place vines to cover multiple blocks in a corner, instead you can always only place it on one side removing the existing vines on the other side(s).
Expected would be that clicking with vines on the other side changes the existing vines to cover this corner as well (this might however be not possible with 3 or 4 enclosing sides.
- Place two solid blocks diagonally to each other creating a corner
- Place vines on one side of the corner
- Try to place vines on the other side of the corner
→ The vines on the other side are removed
Either wait for the vines to spread to the other side(s) or place vines above / below and wait for them to grow there.