So, I am a Vampire - yet, after defeating the moth priest, using the vampire seduction power, then feeding on him, the quest just doesn't progress anywhere - he just stands there saying something along the lines of "If you think I'm going to help you, you are mistaken", everytime I try to open a dialogue.

I have tried loading the game before I entered the cave where he is, and doing the whole thing again multiple times, but no cake.

I am literally stuck there.

Any help would be appreciated.

Edit - I've tried using a pacify scroll on him (my character is not a mage) as a workaround I picked up on another forum, but the moth priest just resists it.

You might have to use the console. I'm doing the Dawnguard side and, after reading the scroll, refused to do anything because he was "busy". Not sure which stage you're at, so hop over to the uesp.net wiki.

I'm pretty sure that this is caused by any mod that edits the Vampire Seduction spell. Try it again without any vampire mods.