Evidence suggest vae indicates 'exit' ('going away') of a season.

Evidence also suggest the season in question is the old year. No evidence has been found which indicates vae was used at the end of any other season.

The 'person' leaving is the sun. The moon ought ideally to be at her full moon phase at new year. If vae occurs among signs of moon, then - presumably - it is the waxing phase which is leaving.

The opposite of vae is a special variant of rima:

Rima has fingers at the top, vae toes at the bottom. Rima has the joint pointing left (backwards), vae the joint oriented forwards.