Boils or vein bulging on vagina. what causes this?

Could be either: A bulging area on the vaginal lips could be a boil or a vein among other things such as cysts and fatty growths. A boil is usually swollen and red around it to suggest infection whereas a vein may or may not be tender. A clotted(thromboses)vein usually is hard and tender. Sometimes it is difficult to distinguish between the two and your best bet would be to see a gynecologist or vein doctor.