Stuart,
Vallejo Model Air is thinned to be airbrushed straight (though I thin it further for airbrushing). Model Color and Panzer Aces are meant to be brush painted, so they require further thinning for airbrushing (if I recall correctly, in the area of 2 thinners to 1 paint in order to pass through most needles, though I personally thin it further). That being said, alcohol is a strict no no. It will turn Vallejo paints to a goo inside your airbrush and cause you endless nightmares.
Though I've always thought it silly that each company makes their own thinners for 'water based' paints, and always thought water could be used, I've recently picked up a few bottles of Vallejo thinners (71.161) and what a difference. Water makes the paint 'wet' (oddly enough) when you spray and I found myself prone to paint spiders or runs doing so. Not so with their thinners, and I thin far more than their website recommends with no issues at all. It sprays on just like a very light coat of paint that takes a few passes to detect and then suddenly starts to build up quickly. I would recommend Vallejo airbrush thinner. Not to be confused with their thinner medium (something akin to a clear coat to mix into the paint and 'increase translucency', might be useful for a topcoat on pre shading). Unfortunately, they do not list the contents of the bottle. I prefer making multiple passes to build up the color, and I do a lot of shading nowadays, so the paint in my cup usually looks close to a wash when I'm spraying. For Model Air I thin at least one to one, more for a final coat.
Hope that helps, and avoid using alcohol to thin, as it's just a big nasty mess. Use their thinner or at least distilled water.