Speaking of Vallejo.
I want to airbrush some of that stuff. Same questions. Can I think it with water? Or what can I use that I might already have? I don't really want to have a million different thinners.
Which Vallejo paint?
Their Model Air line of paint is formulated for airbrushing, and they have a number of items in the Model Air range, specifically to maximize success in airbrushing. Those are adjunct products like thinner, retarder, etc, etc.
Their Model Color line of paint is formulated for brushing by hand, and is popular with figure painters for that reason.
This is not to say that you can't use Model Air for hand-brushing, and can't airbrush Model Color. There are those who do one or the other or both. It's just to note that you may tend to get optimum results if you airbrush Model Air, and use the other Model Air products.
Personally, I have not used Model Air. I have other paints that I already had when I started airbrushing, and they work just fine for me. Tamiya's acrylic paints, Model Master (discontinued, but I have a stash) and Testor enamels, which I thin and shoot very easily with mineral spirits, and Mr Hobby and Gunze-Sangyo lacquers, enamels and acrylics.
I do use Vallejo Model Color for hand-painting figures. I use a wet palette and water to thin it, and it works just fine. Some will say to use distilled water, but I use plain ol' tap water, and I'm happy with my results.
As far as having a lot of different thinners goes, I hear you and sympathize. I don't like to have a lot of different proprietary thinners and paints, either. I do use Tamiya's own proprietary acrylic thinner, X-20A, with their acrylic line of paints (the X/XF series), because after trial and error, I found I get my best results using their thinner with their paints. That's as far as I've gone. All of the other paints in my paintbox can be thinned with water, generic mineral spirits, generic lacquer thinner.
For airbrushing, Tamiya acrylics thin well for me with generic lacquer thinner, too.
And I like to use a wet palette with my water-based acrylics. It helps ensure a consistent.....consistency

, and with the palette closed, I can keep a batch of colors from one session to the next. The drawback is that I can only use water-based paints with it. But that's all right for me.
Hope all that can be of use to you!
Best regards,
Brad