After having spent half an hour looking for a drawing program for vector graphics on the Mac, it occurred to me that I already had one: Keynote .
Obviously, it is aimed at creating stunning presentations, but there is no reason why it cannot be used for any other kind of drawing. When finished, a drawing can be exported as an image file (JPEG, PNG, or TIFF) or simply grabbed from the screen with a screen shot (just use shift-cmd-4 and draw a rectangle).
Keynote is bundles with all new Macs but it is also available separately from the Mac App Store ($20). This price compares well to other graphics applications like OmniGraffle (standard $100, pro $200).