The important thing is the Type of card,(Creature, conjuration,incantation,ect.) So even though Wall of thorns has the living trait, its card type is Conjuration. Just because it is living does not make it a creature, so,
1.can I heal them? It the spells target is "Creature"(and so far they all are) then no. Its a conjuration.
2.does the plant consider (it self) as a creature and (a) plant simultaneously? Creature is a card Type, while Plant is a sub-type.
3.Can Vampiress heal herself after she dealt damage to plant ? The trait for Vampiric states: If you deal damage to a living creature. So I would say no.(I'll admit, I had one attacking my wall of thorns in a game, and allowed it, but now after reading it again, I would say no.As with Bloodthirsty, is says "Target creature"
So basically,just because a conjuration is living, it does not make it a creature. It all comes down to card types, and targeting.(Or legal targets.) I hope I have been of some help, and have not just served as more of an confusion!