I mainly use Vallejo paints, but I don't use either their varnishes or their washes. However, I wouldn't expect any problems when these are used over their own paints. All I can suggest is allowing a longer drying time - a lesson I have just learned the hard way about Vallejo primer! I thought 24 hours would be enough, but evidently not. So 12 hours may not be enough for the varnish. Also, how long are you leaving the paint before varnishing it?
FWIW, I use Pledge floor polish* as a protective coat over any paint, followed by washes made from oil paint & white spirit. I have yet to have any problems with damage to either the Pledge or the paint underneath. You can pick up a set of oil paints very cheaply at a branch of The Works & a bottle of white spirit at Wilko's, if you're interested.
I hope this helps.
Hutch.
*"Pledge multi-surface polish"
Vell, Zaphod's just zis guy, you know?
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