Hello!
I know that problem, it happens to me too, sometimes. I'd say two things to watch for would be to stir the flat clear throughly, to make sure you spray some of the stuff that tends to accumulate on the bottom of the jar. And then, when spraying, try to apply dry, misting coats. Sometimes the flat is much less pronounced when you put on a "wet", heavy coat. I don't relly know Vallejo, but I have used Testors (that has a tendency for yellowing), Humbrol (this one can play trick on you and turn white right after drying - so make a test before application!) and Gunze (quite good clear flat). Hope it helps. Do you have a plan for your next build? Good luck and have a nice day
Paweł