Vallejo Model Color paints are very thick in the bottle. They must be thinned, either with Vallejo's proprietary thinner or distilled (or demineralised) water for airbrushing. You will need to thin at least 3:1 (thinner:paint) before it will flow smoothly through your airbrush.
When brush painting, again, you will need to thin the Model Color paints for best results. They brush on very nicely and they "layer" very nicely indeed. If you do brush paint with these paints, consider picking up some quality brushes - the "$2.00 per dozen" variety found in "dollar store" type places don't cut it.
The Model Air range are considerably thinner and will airbrush out of the bottle, but you will get much better and smoother results with additional thinning.
I've used Tamiya acrylics since they hit the market some 25 years ago, but I recently got hold of my first few bottles of Vallejo's, and I think they're great..