So Riggs is a former Special Forces soldier
I've always liked to imagine that the Die Hard and Lethal Weapon films take place in the same universe. In Die Hard 2 the villain, Colonel Stuart, and at least some of his officers and soldiers are former Special Forces members who presumably fought in Vietnam, just like Riggs did.
Maybe Riggs served and fought alongside Stuart!
And as many have pointed out over on the Die Hard 2 forums, the fight on the wing of the plane at the end of that movie would certainly have been a much more even match if it had been Riggs fighting Stuart instead of McClane.