This is what you need to do. Most of the tension is because we overthink the event too much before the event itself. So you should prepare the viva and when u feel you have enough preparation for it then you should COMPLETELY stop thinking about it. Like they say the fear of event torments you more than the event itself. So you should do everything you can to get the viva out of your mind and assure yourself that you have the preparation for it. Then during the viva try to take deep breaths before going in and youll be fine. Make sure to say everything you wanna no matter how much you stutter. Not saying things make u feel worse afterwards