I have found the setting solution (vinegar) doesn't have much effect, except to make it difficult to slide decals around to position them, after you apply them. I soak the decals in water with a tiny bit of dishwashing detergent to eliminate surface tension, so they can be easily moved around before they dry.
After I let the decals dry thoroughly, if they need to be settled down to conform to panel lines, contours, or whatever, I will use the solvent solution, carefully and quickly brushed on, so as not to tear the decals. It works rather quickly, so brush it on, or simply use an eyedropper, and leave it alone overnight. Also, I have found that Testor's metallizer sealer works on decals much the same way as the solvent does, but I have only used it over metallizer, so test first before you use it over some other kind of paint.