Is it okay to do a tattoo after a vein treatment?

It is best to wait until you heal from your venous ablation.  After the procedure there is often some tenderness, slight swelling and bruising.  Tattooing over the area would not only be more painful but potentially risk infection also.  Just wait several weeks until you are fully healed and then get the tattoo.

As long as you don't have any bruising or trapped blood (this would feel firm under your fingers), then it should be fine. 

Vein treatment may include endovenouos therapy that heats the vein EVLT or radiofrequency, Sclerotherapy, and Phlebectomy and the goal of the vein treatments are closure of the vein and the redirection of blood flow. After a vein treatment you might experience swelling, bruising, and hyper-pigmentation until the fibrous tissue from the vein that was treated gradually incorporates into the surrounding layers of the skin. Some patients heal faster than others, consult with the specialist to make sure you are ready and completely healed before any other procedure on the skin including a tattoo. 
Best, 
Dr. Nguyen

It's important to have the treated vein heal before placing a tattoo needle over it. Your risk of infection and scaring and poor results is increased with the trauma of a tattoo placement. 

I would wait until the vein procedure is healed before getting a tattoo.  It may take a few weeks for your bruising   from the vein procedures to go away.  You don't want to have a tattoo on top of the vein procedure and risk infection in the area that you just had the veins treated in.  I would wait until the vein procedure is healed