Multiple, thin coats area best with Vallejo. I thin my Vallejo Model Colors around 1:1 using either their proprietary thinner or...
A home made mixture of 1:1 ordinary water and artist acrylic matt medium to which I add 5% by volume, each, artist acrylic flow enhance (aka "flow aide") and an artist acrylic drying retarder. This home made mixture is many times less expensive than the Vallejo proprietary Model Color thinner with the added advantage that I also use a lot of artist matt medium as a light duty adhesive (on ground work, etc.)
The brand names of the ingredients for this home made mixture don't matter. Golden works with Grumbacher works with W&N works with... I've never found any of the regular artist acrylic paint products that were not compatible across brands.
I usually need two or three coats of thinned Vallejo over a white or light gray primer coat. Using thin coats also preserves details.
Remember that unlike hobby enamels, you cannot brush out acrylic paints once they start to dry. If you do, you'll get lumps and bumps from the clotted, semi-dried paint. No amount of thinner on your brush will re-dissolve acrylic paint that has started to dry.
So, apply the paint, allowing it to flow onto the surface and leave it alone until it dries. If you need subsequent coats, allow each previous coat to hard-dry before applying the next one.
HTH
