First, unless the subject window is flat glass, merely taking a picture and/or taking measurements of a three-dimensional object will inherently be inaccurate. You need to make your own template by taping a sheet of paper to the window in question and traces its outline with a sharpie. Then spread this template on a flat surface and take a picture of it. Now you have properly rendered a three-dimensional object in two dimensions. Import the photo into your software, make it full size, and layout your image on it. How to make it full size? Lay a 16"x24" framing square on it when you take its picture. Import the image. Create a rectangle that fits exactly over the framing square. Map the image into the rectangle. Make the rectangle 16"x24". Unmap the image. There, as if by magic, you have a full size two-dimensional image of the rear window.
Secondly, don't fret overmuch about rear window wipers. They don't wipe all that often and they won't do a whole hell of a lot of damage to whatever vinyl you put on the window. The vinyl will be lost in the mists of time before the wiper blade makes sufficient passes over it to do it grievous harm.