Well it all depends on taste methinks. First, let's get one misconception out of the way; Any paint can be made to go through an airbrush. It doesn't matter what brand it is - As long as you thin it down, it will
go through the brush.
The best and cheapest way to thin them down, is to use de-mineralized water. If you thin the paint down to roughly the same consistency as low-fat milk, it will go through the brush.
If you buy the airbrush line from Vallejo, the only thing they have done is thin it down for you. It's easier to start off with, but the colour variant is very limited. Their metallics are good, but you will find that you have very few colours to actually pick from. Black, White and such can be easily be bought in the airbrush line though. Whether or not they need to be thinned additionally, depends on the colour. Their white needs additional thinning in my experience, but that colour sucks ass in general for airbrushing.
Personally, I use a variety of Vallejo, GW and Pł. I thin them down as needs be and buy little bottles where I can make pre-mixes of colours I use a lot. Heck, I only use Pł paints for my plasma effects, and they are not made for airbrush - They work just fine though.
In short, there's no need to switch out your entire paint collection, just because it goes in the airbrush - Just thin them down and experiment. Choose your favorite line and go for it