I'm with Madmike on this one - use Tamiya (or any other) primer, and you'll have no trouble. I've had limited success with applying Vallejo directly to plastic, but the metal wargame miniatures that I have primed and then painted are a joy.
I didn't wash the plastic of the parts I've tried painting so far, so I've been 'rewarded" with flaking and chipping. If I had primed it, I am sure I wouldn't be having this trouble now.
One thing for sure, the Vallejo paints have made brush-painting enjoyable again - no stinky enamel and thinner fumes, and best of all, no brush marks!!
Good luck!!
Tony