What does viva success look like to you? What’s the outcome that will make you happy?
If you set it as getting no corrections, or finishing within a certain time limit there may be nothing you could do to be successful.
If you try to be perfect, responding to questions quickly or with perfect paragraphs of ideas and arguments, you will almost certainly fail.
If you define success as doing your best, being prepared, being switched on and ready to engage with your examiners then you’ll have a goal you can achieve.
You get to choose. What will success at the viva mean to you?