I have found that a couple drops of Vallejo's flow improver and a couple drops of water works wonders for spraying Vallejo and AK paints. Maybe more flow improver than water but those two ingredients I have found to be essential for smooth spraying. I can spray with as little as 8-12 psi with that mix.
IPA, Tamiya thinners, lacquers, etc. are too "hot" for those paints and tend to "curdle" the paint rather than thin it.
Hope this helps.