First - welcome back to the hobby! Just polished off a (far larger) Spit myself - you may want to consider tossing it into the Spitfire Group Build that's going on right now.
Second - you'll get tons of different recommendations for thinning Vallejo. My theory - it depends on climate.
I've never had any luck thinning their stuff or anything else for airbrushing with water. It sprays like water, which means it spatters and runs everywhere.
I've used Windex with some success...but the thing is, Windex attacks the paint, too (this makes it EPIC for cleanup), and the ammonia can damage some airbrushes. That and if you get tip dry and build a clog, get ready for all the bubbles!
There are two thinners that I've found work fantastically well with Vallejo.
The first is Future. Lays down nice and smooth with a gorgeous semi-gloss, very tough surface.
The second is Testors universal acrylic thinner. I've only discovered this one recently, to be honest, and the only airbrushing I've done with the Testors/Vallejo mix has been Vallejo Dark Rubber on some Panzer wheels this morning, but it went on with no problems at all. Great when you don't really want to deal with knocking down a glossy surface.
In both cases, I use Windex for the initial rinse or two, then flush with water.
Best of luck! And if it's the Paasche H you've got - fantastic workhorse of a brush there!
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com