You will be vulnerable in the viva. In a way, you have to be.
Your examiners have to ask you questions. But they are carefully asking questions. They’re not trying to hit a weak spot; they have to ask you questions, they have to listen to your responses, there has to be discussion. In doing so, they may hit a weak point, either in your work, your knowledge, your understanding or something else. But that’s not their intent.
You have to put yourself in a position where you’re open to questions. You have to put your thesis out there so that it can be questioned.
You can’t be invulnerable. You have to be vulnerable. For some people that’s one of the scariest things you can do.
What can you do? Acknowledge it, but don’t just worry about it. Work on it, maybe work through it. You can’t build a suit of armour that covers you, but you can – through preparation – make a more confident way of being open to questions and discussion.