Wear whatever helps you present the best “you” that you need in the viva.
If you feel you need to communicate you’re serious, or you know your stuff, or you recognise this is important then wear something that shows this. If you need something to act as armour to protect you, or even a superhero costume then pick something that helps in this way.
If you just want to be really cool then go for it!
In any situation, not just the viva, what you wear can send signals to others about who you are, who you want to be and what you think about the situation.
Of course, you pick up on those signals too. You can reinforce the truth you want for yourself.