Not just a good viva: the best.
Make a list of all of the conditions of the best possible viva for you. However long it is, now make another list of the things you could do to help it turn out that way.
You can’t control everything, so try to stop worrying about the things you can do nothing about. There are plenty of things you can do to help yours be the best viva for you.
Make your lists and get to work.