Helleu?

It's because Esther STILL used the same spell to create him (thereby making the white oak tree the "one true source" of their original power).  So afterwards it gained the same affects/effects.  The only difference is she linked his life to Elena.

katie in vampire diaries blood or my girl caroline forbes my bff  katie lynn dubois  

This is clearly a blooper. Alaric should have been immune to vervain.

It was stated by Esther's mentor Ayana that the Spirits would turn on them. After Rebekah also mentioned that the Spirits turned on them by making the plants burn them. So i'm guessing it's the same with Alaric. Esther created him into an Alternative Original, and the Spirits turned again by weakening him by the Sun and Vervain.