Kyle and Dox
I have to disagree with some of the things Kyle was talking about in panels 7,8, and 9. I think in order for the gag to play out, you would have to remove panel 7 and replace with on similar to 8 and 9. Although 7 is a nice rendering, it is still tough to tell if the expression is of frustration or anger because, well, he's Vader, the ultimate poker face.
I think this gag works better if it is continued past the initial panels of the bill returning. It should be a slow burn rather than a quick 3 panels. The reader will quickly look over panels 4,5, and 6, but then if the same gag is continued with a simple camera change and minimal hand movement by Vader, then it builds the tension and explains the eruption of anger towards a simple soda machine. Although the Storm Trooper is a nice touch, that's not the gag. The emphasis has to be put on the tension between Vader and Soda Machine.
Also, with panel 7, aesthetically speaking, I think the blacks of Vader's helmet would be too heavy next to the other panels which rely more on thinner lines. There's would be a nice symmetry or rhythm of panels before the jump to Vader becoming frustrated on the next page.
Just my two cents...