A Gamble
Simply hoping you will succeed at the viva is leaving what happens up to luck. It’s not wrong to want to succeed, but you mustn’t believe that passing is only due to whether or not things just happen to work out.
Luck and hope isn’t enough. You have to act.
Do the best work you can. Write the best thesis you’re capable of. Prepare when you need to and build yourself up ready for the viva.
You’re not gambling when you take these actions. You’re not leaving things up to chance. You’re readying yourself for success.
You’re taking steps to get closer to what you want and the odds are good that you will succeed!