The Same as Any Living Creature
A dhampir would be healed by Sap Life because they are a living creature (despite their closeness to undeath), and because Sap Life is not a positive healing effect.
Here's from the ancestry description:
Despite being living creatures, dhampirs respond to positive and negative energy as if they were undead
And from the negative healing ability dhampir's have:
A creature with negative healing draws health from negative energy rather than positive energy. It is damaged by positive damage and is not healed by positive healing effects. It does not take negative damage, and it is healed by negative effects that heal undead.
Sap Life doesn't have the positive trait that many other healing abilities like breath of life and lay on hands do, so a dhampir's negative healing wouldn't prevent it from working and they would be healed just as any other living creature would.
A 1st-level harm with a roll of 5 would heal the dhampir for 6 points as you suggested, as long as any living creature also took damage from the spell.