First tip - For figures, prime first with either flat white or a flat light gray. You'll get better, more consistent coverage and truer colors over a white primed surface than on bare plastic.
After that, use multiple thin coats. Vallejo comes out of the bottle quite thick, and if you use it like that, it will leave heavy brush marks, bumps, and ridges.
I thin my Vellejo Model Color about 50:50 for hand brush painting.
Although straight water will work, as well as Vellejo's proprietary thinner, I now mix up my own thinner with half ordinary water, half Vallejo thinner and then add about 5% by volume each acrylic retarder, an acrylic flow enhancer and Vallejo Matt Medium. I use this mixture at about 50:50 with the Model Color paints.
I mix up a 2 oz dropper bottle of this thinner whenever I get short and dispense it by the drop to thin my paints. A bottle will last a long time, so it sounds like a lot of work, but it's actually very easy and quick to use.
For mixing, I use the little paint cups with snap-on lids (the same kind that come with paint-by-numbers pictures). You can buy these by the bag full at any large craft store, like Michael's or Hobby Lobby. I use a permanent marker to write down the Model Color numbers on the lids. These will keep the paint good long enough to paint several projects. I sometimes wash these cups out when I'm done, and sometimes I just throw them away. Depends on how motivated I feel at the time.
Next tip - Keep your brush clean as you paint. After every couple of brush fulls, swish the brush around in clean water to rinse it off. If you get pigment build-up in the bristles, stop and do a more thorough cleaning. Paint will not flow from bristles that are clumped together.
Next tip - Use a long bristle "liner" brush instead of a short bristle round or bright. The liner brush will hold more paint, stays cleaner longer, and is quicker to clean when it does start clumping.
(I use 20x0 sable liners for almost all my acrylic detail painting - figures and vehicles.)
Final tip - Clean your brushes well when you're done with a painting session.
I love Vallejo Model Colors. They are some of the best hand-brushing paints available. They're well worth learning to use.
HTH,