I use Vallejo acrylics almost exclusively which makes me no expert at all but I do have an opinion.
I never had much luck spraying Vallejo Gloss Varnish and I'm not so sure it was ever really meant to be sprayed. It brushes on quite nicely, BTW.
If properly thinned, I think a small spray gun would be more in order to spray this stuff than an airbrush. Or at least a needle/nozzle at maybe .5mm or larger. And probably 20-30 psi which may be way higher than you're used to.
I don't think thinning it with distilled water is a very good idea, I'd use Vallejo's own airbrush thinner. And by all means don't be using Tamiya X-20 or any alcohol.
Hopefully someone will read this who has some better ideas how to spray the stuff.