Another thing I would suggest when using Vallejo Model Color paints is to use a primer to improve adhesion. I just airbrush a coat of Tamiya acrylics as a primer for Vallejo.
Jenny's suggestion of a "wet palette" is a good one. You can buy one "ready made" as Jenny suggested, or make your own ising a small, flat food container, a kitchen sponge (or piece thereof) and a small piece of non-stick baking paper. Even when using small amounts of paint, eg. one or two drops, it will keep it usable for a week or more.
Jenny: I don't know how much you're paying for replacement sheets for your wet palette, but you can simply use the aforementioned baking paper from the supermarket. It works very well.
PS: Remember to shake the paints well. Shake till you think your arm is about to drop off, then shake it some more. Don't try to shortcut this step. (ok, so you can pop the dropper tip off and stick a stirrer into the bottle if you want to.)
PPS: Invest in some quality brushes, preferably red or Kolinsky sable artist's brushes, or Taklon/Golden Taklon (synthetic) brushes. Sure they are more expensive than the cheap "dollar store" or average hobby store brushes, but they are worth it, and properly cared for, will last for many years.
