Can you bruise a vein?

Yes. Most bruises are due to leakage from damaged veins when they are struck or crushed. A blow sufficient to damage a vein is likely to cause leakage which would be interpreted as a bruise.
Bruising a vein. A bruise which results in a black and blue mark on the skin is due to an injury to a vein which bleeds a little under the skin to cause the discoloration. So, in effect, the vein has been injured resulting in a small amount of bleeding leading to a bruise.