Prominent Forehead Vein Appeared One Week After Botox

if this followed the Botulinum Toxin (Dysport or Botox) injection, I supppose some form of muscle imbalance could potentially constrict the vein with activity but I would find that highly unusual.

Botox treatment of the forehead glabellar unit between the eyebrows normally doesn't produce more prominent veins and it is not easy to explain such a reaciton.  Rarely a vein is injected and this can cause a clot and inflammation in the vein which may make the body get rid of the vein. Before that happens, the sclerosis of the vein causes firmness and the area above it swells more as the blood doesn't travel downward below the clotted area. No long term problems occur from this and the vein eventually dissolves. This is all conjecture. See your doctor for an evaluation as time progresses.

A vein appearing after BOTOX may just be unmasked now that the muscle has been relaxed. When a muscle has more tone, it may press on a vein keeping it from being prominent. Once the muscle is more relaxed, that vein may be able to fill up more with fluid and become more visible. As the BOTOX wears off, the vein will go back down. For those kinds of veins, you can do sclera therapy or surgically remove it.