Did you find out from someone that this *is* a mold issue. I am in a 10-year-old house and have discovered the same problem. It was not visible when we did our home inspection in summer. Now it's winter, and the vinyl nearest to both the patio/deck doors and electric heater is going purple in places. Seems to be worst if we place anything on top the vinyl floor, whether it is very light or heavy, it then discolours in the whole shape of whatever was covering the spot.
I have *no* idea how to go about dealing with this. What kind of contract person should I call in to have a look at this?