Well, first I have to distinguish between "Dhampyr" and "Dhampir", just because it can be confusing.
Dhampir: They're "mortals" born of one Thin-Blood (ONLY 15th generation) and a human. They do not suffer the Curse of Caine, but they can't develop disciplines up from one. You can find information of creating Thin-Blood vampires and Dhampir characters on "The Time of The Thin Blood".
Dhampyr: They're almost the same as the Dhampir, but the difference is that they are born of the Kue-Jin, not the Thin-Blood Vampires. As Jadasc had said, the information for creating a character can be found in "Half-Damned: Dhampyr".
There are no rules for V20, but you can just use, in the case of the Dhampir, the character sheets for the V20, as long as Dhampir are almost the same as a Ghoul. In the case of Dhampyr, I found the sheet for V20 version, you can find them here*. There are no rules, but an actualized sheet.
*This comes from MrGone Sheets, a master god of world of darkness sheets.