I am now a Quarra vampire, I've done a whole bunch of quests for my clan, but now I wish to become mortal again.

The problem is I really can't remember how. An age ago, I read somewhere in the game about an Ordinator who got healed and about some expert in Vampirism in Vivec, but I forgot all the details.

Any clues will be very apreciated...

You need to speak to Bal Ur (aka his statue in a Daedric Shrine called "Bal Ur"). Bal Ur's place is north of Suran. Levitate over mountains north of Suran or swim up river and walk south.

PS: The cure ain't free, but of what meaning are the lives of a few more innocents to a Vampire ;)