I've never used (or felt I had to use) primer with Vallejo since standardizing on them over 4 years ago...but that's just me.

IF your airbrush has a regulator you can cap with a finger and backwash air, problem solved.  Mine doesn't so I either mix in a cup first and load the AB, or just put a little too much thinner in first knowing that even if I mix well in the AB cup I'll blow a little pure thinner out at the start.

PS, I know everyone here thins ModelColor, and I am among them, but for my money ModelAir just plain works better so I'm gradually converting my paints over.