The tyrant's focus ability Touch of Corruption states the following:
You infuse the target with negative energy. If the target is living, this deals 1d6 negative damage (basic Fortitude save); on a failed save, the target also takes a –2 status penalty to AC for 1 round.
If the target is a willing undead creature, the target instead regains 6 Hit Points, and if you are the undead target, you also gain a +2 status bonus to damage rolls for 1 round.
The Dhampir heritage doesn't appear to make the character actually undead - they just gain negative healing. If a Dhampir tyrant casts this spell on themselves, which of the following is the case?
The Dhampir is still a living creature, so the first part of the spell is triggered. They take 1d6 negative damage, meaning they heal for 1d6. However, they must then make a fortitude save or receive a penalty to AC.
The Dhampir is healed for the full 6hp, and gains the status bonus to damage
RAI, I believe the second option makes sense, and it's certainly the ruling I would take as a DM if a player did this. But RAW, it appears that the Dhampir will basically shoot themselves in the foot if they use this on themselves, as the Dhampir only gains negative healing "as if" they were undead, but they are actually still living.