Oh Henry!

[rbarata]

The lack of sun and outside atmosphere?

I don't grow Vandas but from experience I know that, just because the plants are indoors, that deosn't mean they are protected. There's always a window with a cold air draft, changes in temp between rooms (I have that when I water my plants), etc, etc.
Plus, in winter, as we heat our homes, humidity gets lower so a watering change must happen.
These are only examples.

[estación seca]

When mine developed roots like that it was always inadequate watering.

I have also read (in real orchid books written by Vanda experts, not on YouTube) Fusarium fungus looks like this on Vanda roots. I don't think you have this problem, since Fusarium is usually a disease occurring with warm temperatures and high humidity.