I am a DM, but i am having difficulty wrapping my head around a suitable answer for my player. Below is the situation and the question following. Please Advise.
Dhampir who uses this spell.
While under the effects of a polymorph spell, you lose all extraordinary and supernatural abilities that depend on your original form (such as keen senses, scent, and darkvision), as well as any natural attacks and movement types possessed by your original form. You also lose any class features that depend upon form, but those that allow you to add features (such as sorcerers that can grow claws) still function. While most of these should be obvious, the GM is the final arbiter of what abilities depend on form and are lost when a new form is assumed. Your new form might restore a number of these abilities if they are possessed by the new form.
Negative Energy Affinity: Though a living creature, a dhampir reacts to positive and negative energy as if it were undead—positive energy harms it, while negative energy heals it.
Would the Dhampir benefit from positive energy while this spell is active?
Yes or no and why?
Infuse self doesn't specifically mention negative energy, so you're looking at the general question of whether a polymorphed dhampir loses negative energy affinity. It's not dependent on a physical feature like teeth, and polymorphing just reshapes you and doesn't affect types, subtypes or anything deeper so no, I don't think polymorphing affects negative energy affinity.