Hey!
I don't think you need to stay away from it at all! I think as long as you frame it positively, mentioning a previous AC can definitely strengthen your answer. You could show resilience and genuine interest in the firm by explaining that, even though you ultimately weren't successful, the experience still confirmed your motivation - especially if speaking to trainees gave you insight into the culture, support, training style, etc.
I'd just avoid focusing on the 'not passing' element, and instead emphasise what you gained from the experience and how it reinforced why the firm appeals to you. Firms are more likely to see returning candidates as a good sign when it is clear that you have reflected on your previous application and stayed engaged with the firm.
Hope that helps - best of luck with the VI!