So I just finished the part where you thrall the moth priest to have him read the elderscroll and I was thinking, that is one of the coolest things i've ever done, and I was wondering if you can get a vampire thrall at any othertime or was it just a one time scripted event?

only this once.

......lame, i figured that was the case, but i was hoping maybe you unlocked a spell later on or something

Yeah that's Bethesda, one of the coolest features in the DLC, one time featured event.