Vase mode appears to start and stop with the presumption of the layer just below and just above being completely level - I get why it would be complicated to presume otherwise.
However, that means the layer height has to start and end at zero-thickness, which is quite challenging in practice. The algorithm for the start/stop of vase mode appears resolve the extrusion height within a single layer. So models with relatively short perimeters will have to rapidly scale the extrusion height, which ends up being quite obvious:
My request, is to enable the layer height adjustment to transition more gradually, based on length of the extrusion or a user-defined number of layers. I assert it's reasonable to assume vase mode would be applied to more than just one layer in the model, and the code knows just how many layers are available to transition the layer height over.
Furthermore, allow the user to set the minimum layer height when switching to/from vase mode. That allows the user to trade off the visibility of the start/stop of vase mode versus the visibility of the 'squish' from forcing the layer height to zero. And rather than abruptly switching from indexed Z steps to continuous indexing, blend the transition so the seam from indexed mode to vase mode gradually disappears.