What would cause a vein to indent on my forehead?

Unclear question: Indent what? Did the vein indent some thing? Did something indent a vein. In any case it does not sound serious. Wish you good health! - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.

Prominent veins?: If you look at the face of actors, you may often find a prominent vein on the forehead. This is just happenstance not unlike the shape of your nose and is of no medical significance unless it has just developed overnight.