Distilled water works great, do you have Model Air or Model Color Vallejo? If it is Model Air it should be thin enough to spray, at most 10% distilled water to 90% paint will do. Model color would need thinning a little more, maybe a 3:1 ratio, it is on the vallejo site somewhere:
www.acrylicosvallejo.com
I just looked and they recommend anywhere from a 3:1 to 2:1 thinning of paint to water.
Good luck! With a little practice I think you will come to love these paints like I have... they really put down a smooth finish and bond fairly well for acrylic. I do recommend getting some of their Model Air thinner which has a retarder and bonding agent in it...