I've used an older version of Mercalli and Vegas 18 in the past and never had any problems. Just installed it and it was immediately available.
I just updated to Vegas 20 and there's currently a Humble Bundle including a 2-year licence for Mercalli v5 (Vegas Plugin not SAL). But unfortunately I can't get it to run in Vegas. Pretty much tried everything; Different installation paths, manually copying the dlls into the OFX folder, even reinstalling Vegas and then Mercalli completely. Other plugins like NewBlue or Ignite are immediately recognized without problems. But Mercalli just doesn't show up no matter what I do. I even contacted their support but they just made some suggestions I've already tried.
So, if someone has the same problem or might know what to do, that would be awesome. Thanks in advance :)!