Did you cure yourself? If so, maybe that's the case.
Yes, I did cure myself in order to do the final attack on the vampires, cause the Dawnguard leader was all like "GTFO you monster" and wouldnt talk to me...so I thought I had to.
Have you tried becoming a VL from Serana again?
Yes, tried that too, the portal still zaps me and wont let me through.