I quit using Vallejo varnishes cause they start to yellow in the bottles over time. When I did, I used them with Vallejo airbrush thinner 50/50 at 20-25 psi. I use liquitex now cause it works well, I can buy it in 16 oz bottles cheap, and they don't yellow. I thin it the same way.
Testors dullcote works well also but its more expensive and hard to get where I am.
Be careful if you use other rattlecan types. Humidity can cause a frost effect.
Hope that helps.
I stopped using Vallejo's gloss for the same reason, but did you ever encounter yellowing with their matte? I've got bottles of gloss less then a year old that are very yellow, but have matte that are over a year that are still pure white.
As for matte spray. If you're looking for actual spray cans, the only good matte I found int he past that was actually matte was from games workshop, but I think they've since changed the recipe, and their sprays seem to cloud more than anyone else (which is the main reason I stopped using them.) If you're more accepting of a low sheen/satin, testor's dullcote is highly regarded and Krylon makes a matte spray that isn't too shiny and is decently priced. If you're talking airbrush, then Vallejo is really the only one out there I have found who has a true matte coat (well, Golden does as well, but it seems to turn to a low satin with handling over time.) I've actually been able to spray Vallejo's Matte through a .33 needle without thinning, although the tip needs cleaning somewhat regularly. If you want to thin it, Liquitex makes a very good airbrush thinner that works well, and you can always try just a bit of water.
Whatever you end up with, using varnish will change the looks at least a little bit on a mini. One of the reasons why Testor's seems to be so popular is because of how it changes the look, some people really like what it does to a paint job.
Also, if you are just spraying the matte directly over the paint, try using a gloss first. Gloss coats basically don't fog so if you put a layer on that is too thick it is very unlikely you will have a problem. As a result, the matte coat you put on only has to kill the shine from the gloss coat and the mini will be fully protected. The gloss coat also makes it extremely easy to put on a matte coat that isn't too thick as it's very obvious when you've covered an area adequately.