Vallejo decal medium

Vallejo's "Decal Medium" looks and sounds like a setting solution (as opposed to a decal softening solvent) applied to the model's surface before the decal is applied.

http://www.acrylicosvallejo.com/es_ES/applying-decals/blog/1/35

I have never used Vellejo myself, but for most brands decal medium describes the stuff you coat old decals that you are afraid might fall apart when they are immersed in water. It essentially coats them with new decal film.  As I say, I have never used Vellejo, but I would assume by that name it would be like other films.