If it is a cartotank, there should be no problems laying it horizontal.
If it is a vivi nova style tank, it depends. I've not owned the genuine vivi nova, but I have had some clones by Smoktech, the Titan or Screw
tanks. Even though they use a threaded screw top, I found if the top is not completely screwed on snug, that they will in fact leak. I first became aware of this when I noticed droplets like sweat on the tank tube's surface.