Its possible they stopped taking vervain thinking she was daggered and klaus was working on their side. For awhile after the season 3 finale, the original drama stopped as klaus and rebekah were daggered and the problem became elena. its possible for them to have forgotten.
Overall, I agree I wasn't a huge fan of the episode. And while I'm probably going to get attacked for saying this, I thought Stefan acted like a baby. Throughout season 3 he literally was pushing Elena towards Damon, pushing her in 3x19 to admit she loved damon, found out in 4x08 she had to have loved damon or she wouldn't have been sired, but when she admits it UNDER COMPULSION