v.co = Vector((1,2,3))

# Note that bmesh is the module named bmesh, not an instance of a BMesh.
# me is an instance of Mesh, again, not an instance of a BMesh.
bmesh.update_edit_mesh(me)

bpy.context.scene.objects.active = bpy.context.scene.objects.active

bm = bmesh.from_edit_mesh(me)

bm = bmesh.new()
bm.from_mesh(me)

bm.to_mesh(me)

me.update()