if you have done the two previous vac schemes, then just say you have been able to consolidate your decision that this is the best career path for you and if you attended an event at the firm, just say you got a great sense for the firm and culture at the event so didn't feel the need to undertake a vac. I have never been asked this and honestly doubt they would do but tbh I think giving a frank and honest answer is absolutely fine. I did two vac schemes before applying for direct TCs and always went to events with firms before applying to gain a sense of their culture so this would be my honest response
To stay true to your answer you could just say the vac scheme experiences have also given you greater confidence in your ability to make a great trainee and think you can more confidently apply for direct TCs now?
I think honesty is the best way to answer but you can leverage that honesty and work it in a way that can be a really strong answer and actually give some insight into you, your character, and what makes you unique