Vallejo "Model Color" paints are very thick - almost gel-like and need considerable thinning before they can be airbrushed. Sorry I can't help with thinning ratios because I haven't started to use them yet, apart from putting a drop or two through for testing.
Because of the amount of thinning required, it is necessary to paint in multiple thin layers - you won't get "solid" coverage in a single coat.
The "Model Air" range paints are considerably thinner and said to be ready to airbrush out of the bottle, but I believe further thinning is suggested.
Thin with distilled water - Vallejo paints don't like alohol and will clump into a gooey mass. (I believe the correct term is "flocculate").
The only other issue is that they need very vigorous shaking to mix the paint in the bottle. The dropper bottle is very convenient for dispnsing paint, but it means you can't stick something in there to stir it. Shake well, or have variable results.