Hi, and welcome to the forum.
Generally a clear gloss coat prior to decalling is only really required where the paint finish is flat. When you apply decals to a flat finished surface, microscopic air bubbles are trapped between the decal and the surface, resulting in an undesired effect known as "silvering".
When you use decal setting solutions, the surface must be glossy before the decals are applied. A "flat" surface is actually "rough" and covered in microscopic pits. When you apply a setting solution to a decal, it will wrinkle and shrivel, but as it dries, will level out and snuggle down onto the surface and any moulded details lying underneath the decal. When the surface is flat (as opposed to gloss) the preciously mentioned rough surface may cause drag on the decal and may prevent it from flarrening out completely before it dries, resulting in yor decals being wrinkled.
Vallejo clear gloss varnish (though I haven't used it) is suitable for the purpose described above.
If you are nuilding a car model, I would expect that you have painted (or will paint) it using gloss paints. If this is the case, you can skip the process of applying a clear coat before applying the decals, as the surface will already be sufficiently glossy.
After your paint has cured and your decals are applied, you can apply a clear gloss coat to "seal" the decals.