You are not overwatering, Bella. In our tropical climate watering twice a day is normal practice. Just make sure your last watering is not too late in the evening to allow for drying out before night falls.Older roots turn brown and that is normal and if there is vegetative growth then your vandas are doing fine.
I water all my orchids twice a day and even mist them in the afternoons during the hot dry season like the one we just experienced. All my orchid friends here and the commercial farms also water twice a day.