Definition of vasotonic:
(adj) :
Affecting the tone of vessels (almost always with reference to blood vessels); that is, affecting the muscle tone of the smooth muscle in the walls of blood vessels, thus influencing blood flow within the lumen.
(n) :
A substance (especially a drug) that is vasotonic, usually with reference to a drug that modifies the tone of vessel walls, such as a vasopressor or a phlebotonic.