There are 2 different types of cards that cause the druid problems, Vines and plants. All Vines are Plants, but not all plants are vines. For example, all of the "Trees" are plants, but none of them are Vines. The Wall of Thorns, while it looks like a Vine, (and may someday become one) is currently only a plant.
Tanglevine and Stranglevine were both Plant conjurations and got added the Vine subtype with the Driud Vs. Necro expansion. The cards may gain or change a subtype over time. Look at Magic the Gathering, many creatures had subtypes changed to allow smoother game play. In MW - Skeletal Sentry for years was not a solider, but with the 3rd printing of the Core set it is now considered a soldier and fits wonderfully in a Skeletal warlord build.
Just so you are aware, and an idea for you - Seedling pod is a vine conjuration. During your deployment phase, it my deploy a plant (remember all vines are plants, but not all plants are vines). You could use the seedling pod to deploy a Wall of Thorns, or the samara tree, to give the seedling pods cantrip.