The vinegar somehow will reduce the amount of 'patches' on the model once the wash has dried up. Without vinegar, you might end up with dark watermark lines, a bit like you would have if you leave your cup of coffee dry up in your cup... dark lines will appear where the coffee level was....
The washing up liquid reduce the surface tension of the water in the wash. Try this: drop some water on a smooth surface and the liquid will form drops. Add washing up liquid to the water you drop, and the water will tend to cover much larger areas without forming regular circular drops. On your model, you want the wash to go all over, and therefore you need to add the drop of washing up liquid to help the wash just do that!