Brush painting is possible, and I have seen some examples of brushed painted models that blows me away! It all depends on the paint. I use Tamiya, and often use a brush to paint small areas with some very good results. However Tamiya is also known for the dreaded "roll up" effect, where subsequent coats rolls up the previous layer. Large areas can be very difficult to paint evenly.
Acrylics are also subject to problems caused by improperly prepped paint surfaces. Cleaning the pieces in mild soapy water (avoid detergents with oils or moisturizers) helps, as does priming the surface. Tamiya makes an excellent rattle-can primer, though be very careful where you spray and use a respirator as it is quite toxic. I also wipe down the area to be painted with isopropyl alcohol prior to painting. Don't over do it, as any remaining residue from the alcohol wash will cause problems.
I haven't used Vallejo acrylics, but have heard from very reputable people that they are probably the best for brush painting.
The brush should be a high quality sable brush as well. Painting is definitely enhanced by quality tools. Thinning is more a matter of preference. I often paint straight out of the jar, especially for small areas, or when using light pigmented paint such as yellow or white. Usually I just add enough thinner to help the paint flow evenly, which is around 10% thinner.