if you have the Paasche VL i'd say that the needle/tip size you'd have the best results with in modelling would be the #3. its been a really long time since i've used a VL, so don't quote me on that. a fine line results when you get these 4 things lined up correctly, determining the correct tip and needle size, the viscosity of your paint in relation to the size of your needle/tip, low airpressure, and the distance between your airbrush and the surface of the subject. all of these factors must be experimented with in order to find the right combination for you and your airbrush. i use the iwata hp-cs which has a .35mm needle/nozzle size, i thin my paint about 60%thinner and 40% paint when doing fine lines, my airpressure is around 8lbs and i'm in really close to the subject, i'd say inside of a 1/4 of an inch or so.
airprressure is going to play a big role in the fine line. frustration will set in quickly without being able to control your pressure. good luck. later.