I'm so sorry to hear about Toby and hope that the vet visit will lead to some sort of improvement to his eyesight. Do you know what has caused the third eyelid to close? Is it as a result of underlying health problems that you are already aware of? It can be as a result of an infection, which can clear up after a course of antibiotics.
FWIW though, I know of a couple of friends, whose dogs who have lost their sight and within a short while they do adjust remarkably well. Of course it does mean that moving things around in the house is not a good idea and you have to adjust your behaviour, by giving verbal warnings and cues rather than visual, so that you don't frighten him-that sort of thing.
Have you checked your pet insurance? Some policies will include cover you if you have to cancel a holiday as a result of your pet becoming ill.
Thinking of you for tomorrow.