There are actually two kinds of varnishes. One on an acrylic base, the other one on a polyurethane (PU) base. Confusingly, Vallejo sells both kinds labelled as matt varnish, so people sometimes confuse the two.
They generally say on the internets that PU varnishes are better (essentially it adds a resin layer on top) but some tend to yellow over time when exposed to sun light (UV). Acrylic varnishes are more common, I think they are more or less acrylic medium which has some additives to make it more resistant.
I read way too much on this topic as I was looking for a durable and the dullest possible finish. Apparently, many people favour Testors Dullcoat but this is not available in Continental Europe, probably because of nasty chemicals in there / EU regulations.
So far, I have tested Army Painter’s Anti Shine Matt Varnish, an acrylic based varnish. Pretty happy with the finish, works also nice if you apply it with a paint brush. Don’t buy these spray cans, it’s hit or miss, many people complain about frosting. I have the impression that it slightly darkens the model, so better apply it in two thin layers. Due to it’s chemical composition you can most likely use isopropyl alcohol on plastic models (it’s often the primer that causes most issues); acetone and other strong solvents can deal with everything, so no worries on that front either.
The other varnish I have is Vallejo’s PU matt varnish. It’s targeted at people who use an airbrush and many say it’s fantastic at that. I don’t have one, so I used a regular brush and diluted it with a bit of air brush thinner. This was probably not a smart idea because mixing it caused quite a bit of foaming which did not completely disappear when I applied it onto the model (it was an orc, so I didn’t mind it that much). I am also not completely sure if isopropyl alcohol will be as effective here; acetone will probably still work. On the positive side, the PU varnish preserved colours really nicely. It is a bit more on the shiny side for a matt varnish but there is nothing preventing you applying a thin layer of acrylic anti shine varnish on top. I think I need to do more testing with undiluted varnish if this also causes foaming.