You will indeed need Zmodeler.
2 ways that I know of to get a vinyl. First is if you want images to be put on the car (such as sponsors ona race car) you will need to make/get a texture of the images. Then UV map it onto the area of the car you want it on. If you want more images rinse and repeat. You can have either all the images in a single texture or in several different ones, or anywhere in between. With them all on a single texture they may not be of the highest quality, but with them all seperated you might not be able to fit them all into the bnk file without removing textures that are needed for the headlights, grills, etc. You will not be able to change the colour of these vinyls in game at the paint shop though.
Another way to make vinyls is to have them as a paintable section. Here you would create a plane/series of planes over top of the existing car mesh in zmod so they are a little bit above the cars body panels. Have a look at cars that are already 2 tone in game, such as the Maserati MC12. In the materials section there will be a PAINT texture and a PAINT2 texture from memory. Most cars will only have the PAINT texture as they are not 2 tone, but if you create a PAINT2 then in the deluxe paint shop you will be able to change the second colour.
After doing either of these then export the model from Zmod and replace it in the bnk with the Modding Tools (make sure you back everything up though!). If you have used custom textures you will have to replace them in the bnk as well, making sure that type of dds (DXT1,DXT5,etc) match between the original and the replacement.
Hope that all makes at least some sense!:cheeky: