Pain is not the 5th vital sign. I know this might be an extremely contentious statement, but it's NOT THE FIFTH VITAL SIGN.
Signs are those things that are objectively observable and measureable. The definition of pain as "whatever the patient says it is, whenever they say they have it" is in direct opposition to this. If "i have 9/10 pain" doesn't mean the same thing to everyone, then it's NOT A SIGN. Blood pressure, respiratory rate, heart rate, temperature, and pulse oximetery are all SIGNS. If I take your blood pressure, with decent technique, and another clinician does the same, the results will be just about the same. Our evaluation of your PAIN however may differ wildly.
This is not to say that pain isn't an important thing to assess, or that it's invalid. it's just NOT A SIGN, and to call it one is, in my opinion, to devalue it, by putting it in the wrong classification.