I was looking for an Eevee in Route 10 Kalos and I encountered a Sigilyph, so I used my Braixen's Fire Spin to give some passive damaging before trying to take it out with Flame Charge.
But...the fiery vortex effect did not happen! The Sigilyph only took the active damage from the turn I used Fire Spin and the passive damage from the vortex effect never happened. This also happened one time when I was fighting a trainer's Slugma but since it was a fire-type, I assumed the vortex does not affect it - However Sigilyph, which clearly isn't a fire-type also didn't take this effect, so I wonder what happened.
Sigilyph had Magic Guard, which prevents indirect damage. This includes the damage received while bound by a move like Fire Spin.
As for Slugma... I'm confident saying this didn't actually happen. There are no Trainers with Slugma in XY, so I think you just misremembered.
Well, probably the Sigilyph had the Magic Guard ability, wich gives immunity to any passive damage, like poison, hazards, recoil and the damage from Fire Spin.