It's after he says whatever sappy, I-need-a-god-damn-bucket line that he says to Roderich in that drawing.

First Roderich is just like '... Pardon?' and Vash is all 'YOU HEARD NOTHING GO TO SLEEP'

Then the teasing begins. The very careful teasing. Teasing Vash is like baiting a lion. If you don't do it right you get your hands bitten off. ouo