I say as long as the middle string type thing is intact the root can grow new tips. I have vandas with root I thought was dead and recently upon checking I found tons of new roots growing off what I had thought was dead root. I've also had rot strip off the top and had a bottom of the root stay nice and green and healthy. I don't cut as fast anymore...I have a recent pic that shows a plant with what looks like dead roots growing nice new healthy roots.
A few vanda I received recently had bad roots, but a good soaking and now they all have new growth like this.