I doubt fusarium... there are some YouTube videos that claim this as the source of many ills... But as ES mentioned, it is very rare outside of the tropics. It would be the last thing I would think of, not the first.
Depending on the exact parentage of your Vanda, temperature is the first thing that I would think of. And humidity. It is normal for some old roots to die, not a concern if you have some that do turn green. The other thing I would think of is lack of water - once a week is not enough water for a Vanda. They need to get wet every day... Water, pour off the water so that they dry, the next day repeat. They also require more fertilizer than most orchids. You could put dilute fertilizer (about 1/4 of whatever it says on the bottle) in that daily watering, and the plant can utilize it. But first... more water, and warm it up if you can.
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