I'm building a Panzer III in German Grey, but to me the chipping isn't like it should be. In the photo I used salt technique with an rusty underlayer. Or am I to impatient and should I apply washes, pigment, dry-brushing to improve the appearance?

Hairspray technique didn't work for me. The paint (Ammo MIG jimenez acrylic paint) was very difficult to scrub off. Any suggestions and tips are welcome. Thanks.

Looking good keep going with the weathering it should bring it all together. I use the chipping solution with ammo paint ,it comes off very easy. Migs chipping solution comes in heavy and light solution,

What should be the next steps, decals and a dark Vandyke Brown oilpaint washing? Is it wise to put a semi gloss varnish first, or can I apply a wash without a varnish layer?

You need to seal the paint with future or a varnish.Next the decals then seal them then you re weathering

Looks alright to me. It might be a question of contrast ? If you lightened your grey up and made the chipping colour darker the contrast would be greater and look more like the chipping you wanted ? (obviously too late for that now).
MIGS stuff is good, so not sure why that not working.Most hairsprays should do the trick too. You are using water based paints when you are trying to do it aren't you ? If you are using enamels it wont work.

Yes, I used Vallejo and MIG acrylic paints and the hairspray does work as I used it before on another model.
How many layers of hairspray should I have applied and how long should the drying time have been for the top paint layer when using the hairspray technique?

I think the look is fine at this stage...
I'd use the sponge technique with some lighter rust and a lightened version (~30% white) of your base panzer grey to simulate lighter scratches plus maybe some metallic grey or even a graphite pencil on the weary edges to simulate that slightly shiny look of often touched steel.
Some light pigments for simulating dust, at the later weathering steps could also help break this up a bit.

PS: I REALLY like the weathered look of that turret bottom under the model! Maybe you can use that for inspiration?

Maybe a bit overdone, but depends of our taste. Anyway looks well done.
You can reload the airbrush with the same paint, or with a light version of the paint and apply it where you don't like.
After the washes and filters and dust, and the flat coat all must be look right

Regards

Alvaro

lighten your German grey & do the edges of your chips. On the very inside of your chips use a pencil or a Vallejo Gun Metal metallic or Tamiya Dark Iron...lightly...very small areas..don't over due it. You can also apply some rust effects...but...not a lot..ie - rusty streaks
After that do your dry brushing, dark & light washes.