Hey!
Congratulations on the AC invite!
As someone who failed to convert a few vacation schemes last cycle, this is definitely something I worried about - however, I wasn't asked about my previous vacation schemes, so try not to stress too much about it!
The key is to frame the vacation scheme positively and reflectively. You don't need to go into details about not converting, as firms understand that there are many factors that go into those decisions. Instead, I would focus on what you gained from the scheme - e.g. the skills you developed, the exposure you had, and any moments that strengthened your motivations for commercial law.
It is also good to show self-awareness by reflecting on what you learned about yourself and what you have done since to improve (based on any feedback you received post-VS). I would focus on tangible steps that you have taken - for example, sharpening commercial awareness, building confidence in group settings, etc. This shows resilience and a willingness to learn, which are essential traits for a trainee!
If you are asked directly about not converting (which I never was!), I'd just keep it honest and avoid sounding defensive. You should focus on discussing how the experience has made you a stronger candidate now, and what you learned from it.
Good luck!!