Same thing here. I simply reinstalled the full Mercalli version and the Vegas plug-in that I purchased last year and everything works. While installing it actually asks which versions of VP you want it installed on and since I have 14, 15 and 16 on my PC, it asked about each version.
I can no longer find the old VP15 stabilizing in the VP16 list. It can only be used if it has been previously inserted in a VP15 project. Was something wrong with the installation? How do I fix this?