The vein between my heel and the ankle hurts badly ! could this be serious?

Might be...: Need a little more history on the type of pain and when it happens. Get checked by the podiatrist. Typically, a vein doesn't hurt directly; it may be impinging a nerve. However, if you can see the vein, it may be swollen; and the cause of that needs to be determined. Also, you may have injured one of the structures beneath. Get checked!

Yes: Any time there is excessive pain it can be serious. See a podiatrist to check it out.