I haven't tried to, but...
Fix up your vehicle .cs file (haven't tried to figure out how), rename it server.cs, put it in a .zip.
Put the vehicle's model and all your textures in the .zip.
Put any sounds or other related files in the .zip.
Make a description.txt, add a description, put it in the .zip.
Voila.
Small note: some of the vehicles (carpet, for example) have a Vehicle_MagicCarpet.cs, and a server.cs that says
exec("./vehicle_magiccarpet.cs");