"Am i missing something?"
just the basic, man.... what a "dangerous shoulder charge" is (often referred just as "shoulder charge") and what "use" of arm means
there's no possible "Vunipola used both arms",.... unless someone could charge with both shoulders at the same time (quite difficult with the head in the middle),.... there's only one arm that matters when talking about shoulder charges and is the one next to the shoulder employed to charge
when in a shoulder charge a player leaves the arm down or near his body is very dangerous (I couldn't find any better image but I think it can be seen)
in this position there's a small contact surface (all the strenght of the hit concentrates in that small part) and bones are predominant in it. This is considered foul play
to"use" the arm means that the player tries to grab the oponent or makes the movement to do it
this image is for a different purpose but just look at the position of the arm respect the shoulder,.... in this position the shoulder offers a bigger contact surface (that soften the impact) and there's more muscle in it. The movement of the arm to separate it from the body and put it in this or similar position is what is called to "use" it
According to Law 10.4:
(g)
Dangerous charging. A player must not charge or knock down an opponent carrying the ball without trying to grasp that player.
(h) A player must not charge into a ruck
or maul. Charging includes any contact made without use of the arms, or
without grasping a player.
Vunipola didn't "use" his arm, it was a dangerous shoulder charge, yellow card,.... and he was very lucky that Barrett was falling backwards and thanks to it didn't hit him in the head
