Where does vasoconstriction occur in limbs?
Vasoconstriction is a phenomenon that can be caused by many factors, from mechanical events (such as stretching) to hormones (epinephrin, neurepinephrin, angiotensin ...). But no matter what is the cause, the end-result is an increase in intracellular calcium concentration in smooth muscle cells (which compose the arteries and veins). So it seems that constriction does not happen in specific points but rather all along the limb. Furthermore angiotensin receptors are found in high concentration in smooth muscle cells [1], suggesting that every such cell has the ability to change it's morphology in order to induce vasoconstriction.

As for your second point, I think the response is as much endocrinological as neurological, since it involves both peripheral and central regulations. (Well, as a side note, endocrine systems rarely have no interaction with central ones).

Angiotensin receptors : distribution,
signalling and function