Vallejo paint question.

You can certainly use Vallejo Model Color paint in an airbrush, it just needs to be thinned a bit more.

Exactly how much thinning, I can't tell you, as Vallejo paints vary in consistency considerably between different colours. I have some which are quite thin and some with the consistency of custard.

If airbrushing (or brush painting for that matter) a nice flat primer coat is recommended as they don't like to stick to styrene very well without it. I just use regular Tamiya acrylic as a base coat. 

Also, Vallejo can be thinned either with water (distilled is probably best), or with their proprietary diluant. You'll have to experiment to discover the best thinner to paint ratio that works for you.

I use a 50/50 blend of golden's airbrush medium and water, and thin with that.  If you need a bit more durability, add a drop or 2 of future to the mix.  Make sure to strain the paint though, vallejo, like citadel, is infamous for little random floating paint goobers!

Chris