I killed the vampire Calvario in the Nerano tomb because I was after the fabled heavy leather boots. I know he is related to a quest though I forgot which one. Did I just break a quest?

I don't think so. You have to kill him along the main quest for the Zainabs.

It is part of the Zainab Nerevarine main quest line. If he is dead then the Ashkhan will acknowledge it and so you have no problems.

Ok, many thanks. No reason for this to stay open.