Most airbrushed sign writing is usually based on a stencil cut by a computer....so in some ways it's easier to use the vinyl. Airbrushing comes into its own when it's used in a more creative way but that , like good hand lettering, requires mastery of the equipment and years of practise.
I have spent years using airbrush as one of my painting tools ....the portrait of Arthur Bray on my avatar was done mostly with an airbrush and I've been asked many times why I don't airbrush my boat. I don't airbrush my boat because I don't think it is suitable....I prefer it looking plain and simple.
I would hate to see any of the classic working boats treated with an airbrush , they are meant to be treated with the respect/artistry of someone like Dave Moore but I can't see any reason why airbrushing shouldn't be used on modern boats. I mostly paint motorbikes.......bicycles .....murals on canvas/walls and I can't see any problems transferring these skills to airbrushing on a narrowboat ....but there again, as it is a labour-intensive skill, it would cost a lot more than vinyl.
Here is a link to some of the stuff I do
www.bikeart.co.uk