I would say Yes.
From Disrupt Undead:
You direct a ray of positive energy. You must make a ranged touch attack to hit, and if the ray hits an undead creature, it deals 1d6 points of damage to it.
From the Dhampirs' Stat block:
Negative Energy Affinity: Dhampirs are alive, but react to positive and negative energy as if undead—positive energy harms it and negative energy heals it.
Since Disrupt generates a ray of positive energy, dhampirs' affinity kicks in and the little bugger is hurt.
As to your other question, unless is specifially says it damages by way of positive energy then I would say no.
Of course this means that 'hide from undead' will not work on a dhampir.
Does disrupt undead hurt dhampirs?
Disrupt undead should hurt dhampirs.
Negative energy affinity states that dhampirs react to positive and negative energy as though they were undead (generally being damage by positive energy and being healed by negative energy). Disrupt undead specifically mentions that it is a ray of positive energy.
Similarly, do other things that hurt undead but are not necessarily positive energy hurt dhampirs?
Negative energy affinity only references positive and negative energy.
EDIT: Ninja'd while I was double checking my answer in the PRD.